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Results of growth of high structural quality gallium nitride single crystals doped with silicon are described in this paper. Dichlorosilane was used as precursor of silicon in the hydride vapor phase epitaxy method. Crystallization runs with different flows of dichlorosilane were performed and compared. One-inch free-standing HVPE-GaN crystals of high structural quality and high purity, previously grown on ammonothermal GaN substrates, were used as seeds. Structural, electrical, and optical properties of HVPE-GaN doped with silicon are presented and discussed in detail. A laser diode built on the homoepitaxially grown GaN is demonstrated.

Study on the sources of stress in HVPE-GaN layer crystallized on 1-in. ammonothermally grown GaN seed is presented in this paper. Characterization by means of X-ray diffraction and transmission electron microscopy is performed. HVPE-GaN samples of high quality and those with visible quality deterioration are investigated on c-plane and m-plane cross-sections. Special attention is paid to HVPE material growing in semi-polar and non-polar directions on the edges of the seed and the growing layer. It is shown that this material generates significant stress leading to a structural deterioration of HVPE-GaN growing in the c-direction.

We use structural and analytical transmission electron microscopy to study the length scale on which InGaN alloy fluctuations influence the optical properties of InGaN quantum wells. We show that compositional and layer thickness fluctuations can be present in InGaN thin layers, depending on the growth conditions and In concentrations. Spectrally and spatially resolved cathodoluminescence performed in the transmission electron microscope indicates that the optical properties of InGaN quantum wells depend on short range order on an atomic scale rather than on medium-range clustering due to lateral segregation.

In this work homoepitaxial HVPE-GaN growth on non-polar and semi-polar GaN seeds was described. Two crystallization processes, in the same experimental conditions but using different carrier gases: N2 and H2, were performed. An influence of growth directions and growth conditions on the growth rate and properties (morphology, structural quality and oxygen and silicon contaminations) of obtained crystals were investigated and discussed. It was shown that the growth rate strongly depends on the growth direction and the carrier gas. It was demonstrated that for the semi-polar [20–21] direction it was possible to obtain high quality and highly conductive (without intentional doping) gallium nitride layers.

Crystallization of GaN by the HVPE technique on an MOCVD-GaN/sapphire template with photo-lithographically patterned Ti mask and ammonothermally-grown GaN crystals (Am-GaN) was studied and compared. Structural and optical properties of the obtained free-standing HVPE-GaN revealed that Am-GaN seeds produced material of much higher quality in terms of etch pit density (EPD), X-ray rocking curves, and excitonic emission than the sapphire-based templates. The crystallization run on the Am-GaN seed, annealed before the growth in H2+NH3 atmosphere, resulted in the HVPE-GaN of an average EPD of 5×104 cm−2, (002) rocking curve width of 22″, and photo-luminescence as narrow as 130 µeV. However, small change in quality was observed when the Am-GaN seed was annealed in N2+NH3 atmosphere prior to growth. In turn, the Hall effect measurements showed that the HVPE-GaN grown on MOCVD-GaN/sapphire template possessed lower free carrier concentration and higher mobility than the HVPE-GaN grown on the Am-GaN seeds. These results were associated to the higher growth rate of the examined HVPE-GaN sample.

We present nitride based laser diodes (LDs) grown by plasma-assisted molecular beam epitaxy. In this work we investigate influence of epitaxial structure on LDs pa-rameters. In particular we compare LDs with and without AlGaN cladding layers operating at 430 to 460 nm. We show results of waveguide simulations which explain the increasing threshold current density of longer wavelength LDs. The PAMBE LD with InGaN waveguides and AlGaN bottom cladding has threshold current density of 4.3 kA/cm2 and slope efficiency of 0.56 W/A. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Advanced Substrates consist of a 200-nm-thick GaN layer bonded to a handler wafer. The thin layer is separated from source material by Smart CutTM technology. GaN on Sapphire Advanced Substrates were used as seeds in HVPE-GaN growth. Unintentionally doped and silicon-doped GaN layers were crystallized. Free-standing HVPE-GaN was characterized by X-ray diffraction, defect selective etching, photo-etching, Hall method, Raman spectroscopy, and secondary ion mass spectrometry. The results were compared to HVPE-GaN grown on standard MOCVD-GaN/sapphire templates.

Gallium nitride layers were deposited on high-pressure-grown GaN substrates containing planar defects. The layer growth above the defects was disturbed and the layer contained terraces. The terraced region was investigated by the molten-bases defect-selective etching (DSE), atomic force microscopy (AFM), and transmission electron microscopy (TEM) methods. The AFM measurement showed significant differences in atomic steps densities in different parts of the layer. The TEM investigation revealed the presence of stacking faults and the DSE showed that the substrate planar defect influences the layer growth. The schema of the defect–layer interaction has been proposed.

Recent results of High Nitrogen Pressure Solution (HNPS) growth of GaN crystals deposited on and separated from 2 inch, and smaller, GaN substrates grown by Hydride Vapor Phase Epitaxy (HVPE) have been presented. The influence of the c-plane bowing in the initial substrate on quality, rate and mode of growth by HNPS method has been analyzed in details.

We discuss various mechanisms of laser diode degradation based on our own experiments and on the available literature data. In most of the cases, degradation of InGaN laser diodes occurs through the increase of the threshold current with almost constant slope efficiency. The threshold current change follows frequently the square root on time dependence. Though this type of behavior has usually been attributed to magnesium acceptor diffusion, no firm proof of such a hypothesis has so far been presented. In contrast, there is an increasing number of reported experiments showing that the most important factor contributing to fast (hours), and medium time (hundreds of hours) degradation is the process of carbon deposition. This process involves photochemical reactions leading to the decomposition of hydrocarbons existing in the laser diode environment. This process resembles very closely the mechanism responsible for 980-nm laser diode degradation and known as Package Induced Failure.

We have studied the properties of InGaN layers and quantum wells grown on gallium nitride substrates with intentional surface misorientation with respect to its crystalline c-axis. Misorientation varied in the range from 0 up to 2 degree. The indium content was changed by using the different growth temperature (between 750 °C and 820 °C) during metalorganic vapor phase epitaxy. With increasing misorientation angle the average indium content decreased significantly. This effect was accompanied by the strong increase of the emission line bandwidth suggesting more pronounced indium segregation. The results of cathodoluminescence measurements show that these effects correspond to different number of atomic steps/terraces existing on the surface of gallium nitride substrate. Very interesting result is also demonstrated concerning p-type GaN layers. With increasing misorientation, the free hole density drastically increases above 1018 cm–3. This improvement in p-type doping is not related to the increased Mg concentration but to the reduction in the compensating donor density. Using this advantage we demonstrate nitride light emitters with improved electrical properties. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A bulk crystal of GaN grown by double hydride vapor phase epitaxy (HVPE) on a needle-shaped GaN single-crystalline seed, obtained by the high nitrogen pressure solution (HNPS) method, was studied by transmission electron microscopy (TEM). Structural defects at characteristic places, such as the seed–HVPE GaN interface, the first HVPE/second HVPE interface, and at morphological imperfections of the seed were characterized. The TEM studies have shown that despite high structural quality of the pressure-grown seed, structural defects are present in the HVPE-grown GaN crystal. A possible reason for strain and structural defects generation could be small lattice mismatch between highly oxygen-doped seed crystal and the HVPE GaN layer as well as between particular sectors of the HVPE GaN layer growing in different crystallographic directions. It was also observed that morphological imperfections of the seed surface may lead to the introduction of the high number of defects extending into the overgrown layer.

Luminescence properties of 100-μm thick GaN epilayers grown by hydride vapor phase epitaxy (HVPE) over three different substrates: high-pressure grown n-type bulk GaN (HP-n-GaN), high-pressure bulk GaN doped with magnesium (HP-GaN:Mg), and free-standing HVPE lifted-off from sapphire (FS-HVPE-GaN), were compared by means of one-photon and two-photon excitations. The contribution of carrier capture to nonradiative traps was estimated by the analysis of luminescence transients with carrier diffusion taken into account. The estimated values of carrier lifetime of about 3 ns and diffusion coefficient of 1 cm 2 /s indicate the highest quality of GaN epilayers on FS-HVPE-GaN substrates.

In this paper we will discuss the application of almost dislocation-free, high-pressure grown gallium nitride bulk crystals as a substrate for the epitaxy of GaN/InGaN/AlGaN laser diode structures. We show that these laser diodes may have very low dislocation densities (even down to 8×10 4 cm −2 ). These dislocations appear during the growth of the laser structure as a result of the combined effect of strain and perturbation of the atomic step flow. We show also that the lifetime of these devices seems to be dependent on operating current via the increasing junction temperature.

In this work, we report on the growth of InGaN layers and InGaN/InGaN multi-quantum wells (MQWs) grown by plasma-assisted molecular beam epitaxy (PAMBE). We show that the incorporation of indium in InGaN layers can be controlled either by the ratio of Ga to N flux or the growth temperature. A method to increase the internal quantum efficiency of MQWs emitting green light at 500 nm by optimizing the growth temperature for the In content of each individual layer is proposed.

High etching rates of HVPE GaN, up to 2 μm/min, have been achieved by photoelectrochemical etching to produce stripes which are 70–100 μm high. These wafers have been used as substrates for subsequent HVPE growth. The surface after the second growth was irregular and needed to be polished prior to defect density evaluation. The dislocation densities measured by the defect-selective etching were 2×10 7 cm −2 and 3×10 5 cm −2 in the stripe region and the low-dislocation-density lateral growth region, respectively. The low-dislocation-density regions formed stripes of about 10 μm.

Single crystals of Mg-doped GaN grown by high nitrogen pressure solution method in different crystallographic directions ([0001], [101̅1], and [101̅1̅]) were investigated in order to determine thermal stability of their electrical and optical properties. Obtained dependences of resistivity, the Hall coefficient and energy shift of Mg-related photoluminescence peak on annealing temperature allow to suggest that incorporation of Mg in GaN is significantly influenced by the direction of the crystallization front.

In this paper we present results of deep-level transient spectroscopy (DLTS) investigations of defects in a GaN-based heterostructure of a blue-violet laser diode, grown by plasma-assisted molecular beam epitaxy on n + -type, dislocation free, bulk GaN substrate synthesized by a high-pressure high-temperature method. The heterostructure contained multiple InGaN quantum well as the active region, which was embedded between two, n-type (Si-doped) and p-type (Mg-doped), GaN wave-guiding layers and two, nand p-type, AlGaN cladding layers, and was covered with a highly Mg-doped GaN contact layer; see [1] for the exact sequence of the layers. The use of a p-n junction of the diode allowed for investigation of both majorityand minority-carrier traps with DLTS. In addition, the kinetics for capture of charge carriers into the defect states, measured by means of recording the dependence of the DLTS-signal amplitude on the filling-pulse duration, provided information on the defect geometry, which allowed distinguishing point defects from extended defects, such as dislocations. Two majority-carrier traps, called T1 and T2, with the activation energies of 0.28 eV and 0.33 eV, respectively, have been revealed in the DLTS spectra measured in the temperature range 80–400 K under typical bias conditions, i.e. under reverse quiescent bias, which was decreased during the filling pulse. On the other hand, DLTS measurements performed under injection conditions, i.e. under zero quiescent voltage and forward-bias filling pulse, revealed one minority-carrier trap, T3, with the activation energy of 0.20 eV. The two majority-carrier traps have been revealed in the DLTS spectra measured under different reverse bias conditions, suggesting that they are present in different parts of the laser-diode heterostructure. In addition, the two traps represent different charge-carrier capture behaviours. The T1 trap, which exhibits the logarithmic capture kinetics, is tentatively attributed to electron states of dislocations in the n-type wave-guiding layer of the structure. In contrast, the T2 trap displays the exponential capture kinetics and is assigned to point defects in the p-type wave-guiding layer of the structure. Additional information on the nature of electronic states of deep levels related to dislocations has been obtained from analysis of the dependence of DLTS-line shape of the T1 trap on the filling-pulse duration. This analysis allowed us to specify the type of electronic states associated with dislocations, which, owing to translational symmetry along dislocation lines, are expected to form one-dimensional energy bands rather than isolated localized electron states. According to the model recently proposed by Schroter et al. [2], which takes into account the rate at which the states reach their internal electron equilibrium within the band, we classify the revealed dislocation-related T1 trap to so-called band-like states in the cores of dislocations.

Plasma-assisted molecular beam epitaxy (PAMBE) has recently emerged as a viable tool for production of nitride blue-violet laser diodes operating at room temperature in continuous wave mode and high output powers [C. Skierbiszewski, P. Wisniewski, M. Siekacz, P. Perlin, A. Feduniewicz-Zmuda, G. Nowak, I. Grzegory, M. Leszczynski, S. Porowski, Appl. Phys. Lett. 88 (2006) 221108]. The present work reviews the current state of the art in this program as well as discusses its future directions. Two elements are given particular attention: (1) the epitaxial growth in metal-rich conditions, which enables effective lateral diffusion of N adatoms at low growth temperatures and (2) the role of threading dislocations in destabilizing the growth front. Low-temperature growth by PAMBE on dislocation-free GaN substrates is instrumental in achieving high performance of optoelectronic structures. The inherent to this process capability of sustaining two-dimensional step-flow growth mode (with straight and parallel atomic steps) at low growth temperatures opens up the way to the growth of strained multilayer structures with no compositional fluctuations and with flat interfaces.

Light amplification features of similar group-III-nitride laser structures grown by metal organic vapour phase epitaxy (MOVPE) and plasma-assisted molecular beam epitaxy (PAMBE) as alternative techniques were investigated. The samples are characterized by very low dislocation density (105 cm?2) that results from the use of high-pressure-grown GaN substrates. Gain measurements were carried out by means of the variable stripe length method. A maximum excitation power of 464 kW cm?2 corresponds to a peak modal gain value of 180 cm?1 for the MOVPE-grown sample and 315 cm?1 for the PAMBE-grown one. Saturation lengths of 350 ?m and 250 ?m measured for MOVPE- and PAMBE-grown samples, respectively, indicate reduced nonradiative recombination compared to heteroepitaxy on foreign substrates. Activation energies associated with InGaN band profile fluctuations derived from temperature-dependent photoluminescence measurements yield values of 41 meV and 22 meV for MOVPE and PAMBE, respectively. Improved band profile smoothness is accompanied by a reduced threshold for stimulated emission in favour of the PAMBE-grown laser.

Optical gain and losses in semiconductor laser epilayers are of profound importance in device optimization. We used both parameters to estimate the potential of InGaN/GaN/AlGaN epitaxial system to be used as a laser structure emitting in a blue region. The sample was grown homoepitaxially by MOVPE technique on high quality bulk GaN substrates obtained in high pressure growth process. These structures are characterized by very low dislocation density (around 105 cm–2). As a result the reduction of a nonradiative recombination is expected which is reflected in reduced excitation pump power and gain saturation processes described by means of saturation length and a product of the latter and a maximum modal gain. To determine gain value and its saturation behavior a variable stripe length method is used employing one dimensional amplifier model. Maximum gain value of 190 cm–1 together with internal losses of 15 cm–1 are determined for the studied structure. Low value of maximum saturation length of 320 um was found in our structures. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this article homoepitaxial HVPE-GaN growth in directions other than [0001] is described. Three crystallization runs on (11-20), (10-10), (20-21), and (20-2-1) seeds were performed. In each experiment a different carrier gas was used: N2, H2, and a 50% mixture of N2 and H2. Other conditions remained constant. An influence of the growth direction and carrier gas on growth rate and properties (morphology, structural quality, and free carrier concentration determined by Raman spectroscopy) of obtained crystals was investigated and discussed in details. For all crystallographic directions a lower growth rate was determined with hydrogen used as the carrier gas. Also, the highest level of dopants was observed for crystals grown under hydrogen. A possibility to obtain highly conductive GaN layers of high quality without an intentional doping is demonstrated.

Room temperature, continuous wave operation of InGaN multi-quantum wells laser diodes made by rf plasma assisted molecular beam epitaxy at 411 nm wavelength is demonstrated. The threshold current density and voltage were 4.2 kA/cm and 5.3 V, respectively. High optical power output of 60 mW was achieved. The lifetime of these laser diodes exceeds 5 h with 2 mW of optical output power. The laser diodes are fabricated on low dislocation density bulk GaN substrates, at growth conditions which resembles liquid phase epitaxy. We demonstrate that relatively low growth temperatures (600–700◦C) pose no intrinsic limitations for fabrication of nitride optoelectronic components by plasma assisted molecular beam epitaxy.

Due to their wide band gap range of InGaN from 0.7 eV to 3.5 eV InGaN, heterostructures are used for optoelectronic applications covering the entire spectral range of visible light [1]. Pyroelectric fields influence the optical properties [2]. Additional strain-induced piezoelectric fields are typical for group III nitride heterostructures. Homoepitaxial growth on single GaN crystals is predicted to reduce the strain. We compare the luminescence properties of InGaN heterostructures on bulk GaN substrate and sapphire. We present the results of time-integrated and time-resolved photoluminescence (PL) spectroscopy, performed on InGaN single quantum wells (QW) with widths of 4.5 nm and 9.5 nm. The structures were grown by metal organic chemical vapor deposition on sapphire and high pressure bulk GaN substrates [3]. Samples deposited on both types of substrates were grown in the same run. This allows us to compare directly the population and recombination mechanisms in thin and wide QW’s on both substrate materials. Si-doped buffer/barrier layers were used to screen built-in electric fields. The PL was excited at 353 nm by the second harmonic wave of a mode-locked Ti:sapphire laser. The temporal width of the laser pulses was 2 ps at a repetition rate of 80 MHz. The PL measurements were performed in a helium-flow cryostat. The detection system consisted of two 0.35 m McPherson monochromators in subtractive mode and an ultra-fast photo detector (micro-channel plate) providing a spectral resolution of about 1 meV and a time resolution of better than 30 ps. Figure 1 shows the low temperature PL of the investigated samples. The QW structures on GaN emit at lower energies than their sapphire-grown counterparts. We suspect that the cause of this is a lower temperature of the bulk GaN substrate during growth (in spite of its better thermal conductivity). The 60 µm thick GaN substrate shows a stronger bowing and worse contact to the heating than the 300 nm thick sapphire substrate. This leads to a more efficient incorporation of indium into the structure grown on

Role of band potential roughness on luminescence decay time and stimulated emission in InGaN quantum wells (QWs) grown by rf plasma-assisted molecular beam epitaxy (MBE) on bulk GaN substrates was studied. A high-photoexcitation regime used ensured conditions similar to those in operating laser diodes. Standard deviation of the potential fluctuations in different thickness InGaN QWs was found to vary in the range of 13-22 meV as revealed by Monte Carlo simulation of localized exciton hopping. A negligible influence of this variation on the luminescence decay time (-700 ps) and stimulated emission threshold (∼30 kW/cm 2 ) was observed. We attribute this insensitivity to the low density of localized states (∼1 × 10 18 cm -3 ) estimated in our high-quality QWs grown by MBE, and therefore, assign extended states to be mainly responsible for the properties of highly-excited luminescence.

By means of hydrostatic pressure-dependent measurements of the electroluminescence of blue/violet laser diodes with In0.1Ga0.9N/GaN active layers grown on bulk GaN it is shown that at conditions close to lasing threshold full screening of polarization-induced electric fields (PIEFs) is provided by injected charge. In contrast, in order to achieve PIEF screening at currents much below lasing threshold, Si doping of quantum barriers is seen to be necessary. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Results of high pressure solution growth of GaN on various patterned substrates are presented. The growth on MOCVD GaN/sapphires templates patterned in GaN parallel stripes and with SixNy mask is described in details. The usefulness of Mo, Ni and SiO2 films as masks for lateral overgrown of GaN is also studied. The results of crystallization on thick HVPE free standing GaN patterned in 20 µm high stripes are shown. The role of stripe edges, as seeds for growth of dislocation free GaN crystals is discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present study of carrier dynamics in GaInN/GaN multi quantum wells (MQWs) with screened electric fields. The PL emission showed clearly resolved peaks emitted from the Ga0.9In0.1N QWs (at about 3.1 eV, FWHM 30-50 meV). Two samples with 2.5-nm-QWs and 4-nm-QWs showed decay-times of 1.1 ns and 2.3 ns. The decay-rates in the barriers were thermally activated, activation energies were 21 meV and 46 meV, for the 2.5-nm-QWs and 4-nm-QWs structures, respectively. Analysis of correlation between rising-rates of the MQW PL and the decay-rates of the barriers PL confirmed increase of transport through barrier. The MQW PL energy changed with time. The phenomenon was due to trapping of excitons on donors or localization in potential minima. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work we present a reliability study of low dislocation density InGaN laser diodes fabricated on high-pressure grown GaN monocrystaline substrates. The aging process was performed under pulse current conditions. Degradation of these devices manifests primarily in the increase of the threshold current. Interestingly, the differential efficiency of lasers remains stable at all times. The aging time dependence of the increase of the threshold current precisely follows a square root dependence. These observations suggest that the degradation results from the enhancement of the nonradiative recombination within the device active layers and is related to point defect diffusion.

High-pressure growth from solution is at present the only method able to provide true bulk GaN monocrystals. In this paper, we would like to demonstrate that in spite of their small, centimeter range size, they can become a technological platform for the realization of high-quality violet and near-UV laser diodes. We used MOVPE technique to deposit InGaN/AlGaN/GaN layers forming a separate confinement heterostructure laser diode. These devices are characterized by a low density of dislocations (∼10 5 cm −2 ) and a high optical output power of 1.9 W, measured in short pulses (30 ns) to prevent the structure from overheating. We will briefly discuss the challenges we face during the process of optimization of these devices.

GaN substrates grown from solution under high N 2 pressure without an intentional seeding are almost dislocation free single crystals in the form of thin (about 100 μm) hexagonal platelets with a lateral size up to 1 cm. They were used as substrates for HVPE which allows very fast growth of GaN in the c -direction. Both n-type GaN crystals (lattice mismatch to pure GaN of 10 −4 due to high electron concentration) and high resistivity GaN:Mg crystals (lattice matched to pure GaN at RT) have been used. Most of the layers were grown on the Ga-polar (0 0 0 1) surface of the substrates. It is shown that at the applied growth conditions, near dislocation free GaN can be deposited only up to a critical thickness dependent on the type of the substrate used. The possible mechanisms of the relaxation processes is discussed on the basis of defect selective etching and micro Raman measurements for the cleaved cross-sections of the HVPE-GaN/pressure-GaN samples. It is also shown that stable (in terms of flat crystallization front and material continuity) growth of GaN on GaN substrates is possible at a rate as high as 500 μm/h. In this way, the single crystals of GaN with thickness exceeding 2 mm were grown. These crystals were sliced along non-polar crystallographic planes. Thus, both {1 0 1¯ 0}- and {1 1 2¯ 0}-oriented platelets have been obtained to be used for epitaxial growth of quantum structures by MBE.

Results of high-pressure solution growth of GaN on GaN/sapphire substrates, patterned in GaN parallel stripes, are presented. The stripe growth in lateral and c-directions is examined and analyzed in detail. A defect-selective etching method is used to determine the dislocation densities in the areas corresponding to the initial stripes and the laterally overgrown material. Some experimental conditions necessary to obtain low dislocation density in the overgrown GaN stripes are defined.

There has been more than a decade since Shuji Nakamura from Japanese company Nichia constructed the first blue LED based on structure of (AlGaIn)N semiconductor and eight years since he made the first blue laser diode (LD). This work gives a survey on the current technological status with green/blue/violet/UV optoelectronics based on III-N semiconductors in relation with their microstructure. The following devices are presented: i) Low-power green and blue LEDs, ii) High-power LEDs targeting solid-state white lighting, iii) Low-power violet LDs for high definition DVD market, iv) High-power violet LDs, v) UV LEDs. The discussion will be focused on three main technological problems related to the microstructure of (AlGaIn)N layers in emitters based on III-N semiconductors: i) high density of dislocations in epitaxial layers of GaN on foreign substrates (sapphire, SiC, GaAs), ii), presence of strains, iii) atom segregation in ternary and quaternary compounds. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We measured the bowing of structures consisting of epitaxial layers on bulk GaN substrates. The bowing is a result of lattice mismatch between layers and substrate. The layers were grown by HVPE (Hydride Vapor Phase Epitaxy) and MOVPE (Metalorganic Vapor Phase Epitaxy) techniques. The substrates are produced using high-temperature, high pressure method. In the present paper, the results of bowing measurements are presented compared with theoretical calculations. Based on the results, critical radius of bowing is estimated. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigate screening of polarization induced electric fields in a group III-nitride based laser diode with InGaN active layers. Via hydrostatic pressure dependent electroluminescence measurements and additional photocurrent spectroscopy experiments, it is shown that the introduction of Si doping with a concentration of 1.10 1 9 cm - 3 in the barriers of the active region leads to adequate screening of polarization induced electric fields.

In this paper the results of directional high-pressure growth of GaN on sapphire/GaN MOCVD templates are described. The use of a baffle plate is presented, in order to obtain the flat crystallization front at the substrate. The GaN growth rate as a function of the applied temperature gradient and time is analyzed in detail. The optimal temperature gradient for the fastest growth is determined. The changes of the growth rate with time are explained. The defect selective etching method and transmission electron microscopy are used to determine the dislocation density in the deposited GaN material. All results are compared to those obtained for directional growth of GaN on pressure grown GaN crystals (platelets).

Contains fulltext : 32393.pdf (Publisher’s version ) (Closed access) In this communication crystallographic and chemical inhomogeneities occurring in GaN single crystals, homoepitaxial layers and quasi-bulk thick epitaxial layers are described. Practical classification of defects as (i) growth-related and (ii) processing-induced is given. On the basis of numerous studies using different examination techniques (DIC and DF optical microscopy, SEM) and methods of revealing (defect-selective etching, PEC etching, X-ray diffraction and TEM), the most dangerous technology steps for the formation of defects are described and the types of defects, which might be critical for performance/operation of the GaN-based opto-electronic devices are indicated. It is shown, that the routine use of a simple defect-selective etching method during the subsequent stages of the technology of GaN homo-epitaxial lasers is helpful in achieving time-effective device processing. (c) 2005 Elsevier B.V. All rights reserved.

High-pressure-grown bulk GaN crystals have the best reported structural quality and the lowest defect density among all available GaN substrates. We report on MOVPE growth, device processing and properties of high-power laser structures on such substrates. In this work, we describe very high optical output power per laser facet of 1.9 W from single LD device with 15 μm×500 μm cavity. The measured value is up to our knowledge the highest nitride laser power reported. Also, the electrical and optical power densities, yet pulsed, are in the range of the highest reported for broad area lasers what confirms that nitride-based wide bandgap structures are capable of handling very high power. From thermally accelerated electrical ageing tests we extracted the activation energy of one of degradation mechanisms to be 0.32 eV. Catastrophic optical damage was not observed up to power density of 40 MW/cm 2 . Demonstrated results show that the availability of very low defect density GaN substrates is clearly the key issue in fabrication of highly reliable devices and high power LDs.

In this work we demonstrate the quality of laser diodes fabricated on bulk GaN substrates. These substrates were obtained via high-temperature, high-pressure synthesis and are characterized by a dislocation density as low as 102 cm−2. The epitaxial structure was deposited using a combination of MOVPE and MBE methods. Thanks to such a low and uniform density of dislocations it was possible to realize high power, pulsed current laser diodes with a total output power exceeding 2.5 W. Interestingly enough, the MBE growth method can produce very satisfying results once the growth is resumed on bulk GaN substrates. This contradicts the common believe concerning the poor applicability of the MBE method for the growth of nitride violet laser structures. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN single crystals exhibiting the lowest dislocation density (below 100/cm 2 ) are grown at high hydrostatic N 2 pressures of 10-20 kbar. Despite small dimensions of such crystals (up to 1/2) they offer a unique chance to construct high power laser diodes and some other devices. However, in order to develop high quality epitaxial structures, a number of steps, different to GaN epitaxy on foreign substrates, must be made. These steps include: (i) surface preparation of Ga-terminated side used for epitaxy, (ii) optimization of substrate thickness, (iii) optimization of substrate miscut, (iv) N-terminated side preparation for back-side contact. This work contains the following information: (i) description how the blue laser diodes on bulk GaN crystals are made, (ii) what is their crystallographic quality (in particular, a very large bowing for unrelaxed structures will be shown- the bowing radius can be as small as 0.1 m), (iii) what are the optical (very low threshold of 2.5 kW/cm 2 for optical pumping) and (iv) electrical parameters (2.6 W (1.3 W per facet) optical power under pulsed operation).

Gallium nitride, aluminum nitride and indium nitride are basic materials for blue optoelectronic devices. The essential part of the technology of these devices is annealing at high temperatures. Thermodynamic properties of the Ga–N system and their consequences to application of high nitrogen pressure for the annealing of GaN based materials are summarized. The diffusion of Zn, Mg and Au in high dislocation density heteroepitaxial GaN/Al2O3 layers will be compared with the diffusion in dislocation-free GaN single crystals and homoepitaxial layers. It will be shown that high dislocation density can drastically change the diffusion rates, which strongly affects the performance of nitride devices. Inter-diffusion of Al, Ga and In in AlGaN/GaN and InGaN/GaN quantum well (QW) structures will be also considered. It will be shown that in contrast to stability of metal contacts, which is strongly influenced by dislocations, the inter-diffusion of group III atoms in QW structures is not affected strongly by the presence of high dislocation density. This is related to the different rate controlling slow process in these two diffusion processes. This feature of interdiffusion processes explains the success of heteroepitaxial techniques in the technology of nitride based light emitting diodes.

The results obtained with the use of pressure-grown GaN single-crystalline substrates allow us to draw the following conclusions important for the construction of In-free UV light emitting diodes and lasers and InGaN-based high-power blue lasers. (1) The application of pressure-grown GaN single-crystalline substrates allows us to grow near-dislocation-free layer structures by both metal– organic chemical vapour deposition and molecular beam epitaxy. (2) The elimination of dislocations leads to highly efficient UV emission from GaN and GaN/AlGaN quantum wells, which is impossible for strongly dislocated structures grown on sapphire. (3) At high excitations (e.g. in lasers), dislocations are also effective non-radiative recombination centres in InGaN-containing structures, so the elimination of these defects is crucial for better performance of blue lasers. In this paper, the optical and structural properties of the near-dislocation-free GaN-based structures leading to the above conclusions are discussed.

Room temperature (RT) pulsed operation of blue (402-427 nm) InGaN multi-quantum well (MQW) structure laser diodes (LDs) grown on bulk GaN crystals have been demonstrated. Substrate crystals were grown with the high nitrogen pressure and high temperature solution method. After substrate preparation the laser structures were grown by the low pressure MOVPE process. The LD structures are fully strained on the whole area of GaN crystals, without mismatch-related defects, being confirmed by X-ray diffraction, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Stripe geometry cavity has a contact area of 10 x 500 μm 2 . The mirror facets were cleaved and coated with reflection dielectric layers.

In this communication, the use of gallium nitride doped with beryllium as an efficient converter for white light emitting diode is proposed. Until now beryllium in this material was mostly studied as a potential p-type dopant. Unfortunately, the realization of p-type conductivity in such a way seems impossible. However, due to a very intensive yellow emission, bulk crystals doped with beryllium can be used as light converters. In this communication, it is demonstrated that realisation of such diode is possible and realisation of a colour rendering index is close to that necessary for white light emission.

The application of the pressure grown GaN single crystalline substrates allows to grow near dislocation free layer structures by both MOCVD and MBE. This is demonstrated by X-ray, AFM, TEM and defect selective etching results showing high structural perfection of both GaN substrates and epitaxial structures. The properties of these near dislocation free epitaxial layers and structures indicate that dislocations are the important factor limiting radiative efficiency of nitrides, especially at high excitations.

We have grown GaN films and GaN-AlGaN quantum wells (QWs) on homoepitaxial substrates, by molecular beam epitaxy using ammonia. Both the GaN film and the QW are found to have superior excitonic recombination properties which are extremely promising for the development of indium free ultra-violet lasers based on nitrides. (C) 2001 Published by Elsevier Science Ltd.

High-pressure measurements were employed to study mechanisms of (i) the 3.27 eV emission band (and phonon replicas) and (ii) 2.9--3.1 eV blue photoluminescence in different types of GaN:Mg samples. GaN layers with low Mg doping, grown on bulk GaN crystals and on sapphire substrates, were used to study the 3.27 eV uv band. This band results from donor-acceptor pair (DAP) recombination (with both impurities of shallow character). For high Mg doping resulting in blue light emission at about 3 eV, a bulk GaN crystal and a layer grown on sapphire were employed. For the all samples the pressure coefficient of the corresponding luminescence bands was measured at 77 K by means of the diamond anvil technique. The energy shift of the 3.27 eV band under hydrostatic pressure is used to determine the linear pressure coefficient characteristic for (shallow) DAP recombination. It has a value of about 35 meV/GPa, close to the pressure shift of the GaN band gap (about 40 meV/GPa). The 2.9--3.1 eV blue luminescence band exhibits much lower pressure coefficients of about 23 meV/GPa. It shows that the initial state of the blue emission has localized character in both types of GaN material and supplies a strong argument against models of radiative recombination in highly-Mg-doped/compensated GaN material that postulate an involvement of delocalized (shallow) electronic states as the initial states.

Oxygen contamination is the main source of free electrons in bulk GaN grown at high-pressure. We used erbium as a promising getter/compensation center to reduce the electron content in GaN. Both optical and electrical measurements indicate the reduction of electron concentration from the initial 6 × 1019cm−3 down to 1 × 1019cm−3. A photoluminescence study proves that erbium is incorporated into the GaN host lattice, and emits light at ∼ 1.5μm.

We performed an infrared study on the highly n-type and semi-insulating GaN single crystals. We also studied epitaxial layers (homoepitaxy) grown on the ( 0 0 0 1 ) N and (0 0 0 1)Ga faces of these crystals. For the n-type GaN single crystals, we show that the ( 0 0 0 1 ) N faces always possess much higher free carrier concentration than the (0 0 0 1)Ga face. Moreover, infrared reflectivity of the layers deposited on both polarity of semi-insulating and highly conducting bulk GaN substrates shows that the incorporation of impurities (most probably oxygen and silicon) is systematically higher in the layers grown on the ( 0 0 0 1 ) N faces. These results clearly indicate that the growth of epitaxial layers on the bulk GaN substrates should be done on the (0 0 0 1)Ga faces of the GaN crystals.

The results obtained with the use of the pressure grown GaN single crys- talline substrates allow to draw the following conclusions important for the construction of In-free UV light emitting diodes and lasers and InGaN-based high power blue lasers: 1. The application of the pressure grown GaN single crystalline substrates allows to grow near dislocation free layer structures by both metal organic chemical vapor deposition and MBE. 2. The elimi- nation of dislocations leads to highly efficient UV emission from GaN and GaN/AlGaN quantum wells which is impossible for strongly dislocated struc- tures grown on sapphire. 3. At high excitations (i.e. in lasers) dislocations are effective nonradiative recombination centers also in the InGaN contain- ing structures, therefore the elimination of these defects is crucial for better performance of blue lasers. 4. The analysis of microstructural and optical properties of the InGaN containing dislocation free structures shows that the main mechanisms of carrier localization in InGaN are not related with the nm scale compositional fluctuations in InGaN. In the paper, the optical and structural properties of the near dislocation free GaN-based structures leading to the above conclusions are discussed.