Search

Thin films of CuGaSe were deposited on n-Si (1 0 0) by rf magnetron sputtering from a stoichiometric CuGaSe2 target. The objective of this study was to characterize the thin film/Si heterojunction for potential photovoltaic applications, evaluate possible candidates for metal contacts and to establish whether heteroepitaxial growth could be achieved, particularly as the mismatch of lattice parameters corresponding to the base of the copper gallium selenide (CGS) tetragonal cell is quite close to that of Si, with a 2.9% mismatch. For this study, Si substrates were prepared by the standard Radio Corporation of America (RCA) cleaning procedure immediately followed by the deposition of CGS by sputtering at a substrate temperature of 600°C. The deposited thin-film stoichiometry and morphology were characterized by Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM). Rutherford back scattering (RBS) analysis indicated a thin-film composition of Cu1Ga1Se1 indicating that the films were Se deficient, although channeling was not observed. The polycrystalline nature of the deposited thin film was established by cross-sectional TEM. An estimated 1.5-nm thick layer likely to be SiO x was observed at the CGS–Si interface. It is believed that this interfacial layer prevented heteroepitaxy CGS on Si. Additionally, circular metal contacts were deposited on the thin films and characterized by capacitance and current–voltage measurements. It was observed that Al and Ag contacts were rectifying, from which the thin-film carrier density was estimated to be ~5 × 1015 and ~7.68 × 1015 cm−3 with Al and Ag contacts, respectively. Au, Pt, W and Cr were ohmic, and Mo and Ni provided semi-ohmic contacts to CGS films.

Current–voltage (I–V) characteristics and electroluminescence spectra of several ultraviolet and blue light-emitting diodes (LEDs) emitting nominally at 380, 400, 430, and 468 nm were studied. These diodes exhibited an Ohmic regime at low forward biases; then the current increased sharply as bias increased. Several changes in the slope of logarithmic I–V plots indicated that, I ∝ Vx. These changes in the slope were interpreted as single-carrier space-charge-limited (SCL) transport across the diode active region. As bias increased the deep states were filled and for the 400- and 468-nm diodes ideality factors of ∼2 were obtained. This indicated that, as bias increased the transport mechanism changed from SCL conduction to recombination of injected carriers in the space-charge region. For the 380- and 430-nm diodes, ideality factors >>2 were obtained, although the observed electroluminescence spectrum indicated substantial radiative recombination. For the diode emitting at 430 nm, several peaks including the major peak at ∼424 nm appeared to have resulted from transitions between the conduction-band edge and deep states, identified from the I–V characteristics, likely to be associated with Zn doping of the InGaN active region. Deep states in the other diodes appeared to be ineffective in the radiative recombination process.

Rectifying contacts to n-type GaN formed with sputter deposited films of Au, Cu, Cr, Ni and Pt were studied using current–voltage and capacitance–voltage measurements. These rectifying contacts were non-ideal as seen from the non-linearity of the semilogarithmic plots of the forward characteristics. Also, the high ideality factors as determined from the approximately linear region of the plots were much greater than one. The non-ideal behavior cannot be understood in terms of thermionic emission simply by excluding the effects of a series resistance and recombination current. This deviation from thermionic emission has been interpreted to be due to space charge limited current conduction in the presence of deep-level states. A simple analysis of these characteristics has been used to identify the deep-level states in the energy range 0.27–0.43 eV below the conduction band minimum and with an approximate concentration between 1015 and 1016/cm3. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Sputter-deposited Au, Pt, Cr, Ni and Cu contacts for n-type GaN films were studied using current–voltage (I–V) and capacitance–voltage (C–V) measurements. These films were grown by molecular beam epitaxy (MBE), heteroepitaxially on the basal plane of sapphire. The contacts were non-ideally rectifying in nature. Assuming that the non-ideality was due to effects of series resistance and recombination current, a computer curve fitting procedure was employed that enabled the separation of these effects from the thermionic emission current, thereby permitting the calculation of the barrier height. An analysis of the results indicates that the barrier heights for metal contacts on GaN are determined by the difference between the metal and the semiconductor electronegativities and substantially influenced by metal induced gap states (MIGS)/sputtering damage induced surface states (SDISS). The concentration of metal induced gap states/sputtering induced damage states was determined to be approximately 2.7 × 1013 states cm−2 eV−1.

An analytical model for the structures commonly employed for the fabrication of blue light emitting diodes (LEDs) based on AlGaN/GaN/AlGaN double heterojunctions has been developed. Insight gained from the analysis was employed to interpret observed current-voltage and capacitance-voltage characteristics obtained from commercially available blue LEDs. These diodes are highly non-ideal with several regimes characterized by high values of the ideality factor ranging from 2.90 - 6.76 indicating that the recombination process cannot be described by the Shockley-Read-Hall mechanism. Logarithmic plots of the forward characteristics indicate a space-charge-limited-current conduction through the active region of the diodes. Observed changes in slope of these logarithmic plots are representative of a high density of "deep-level states" in the active region of the diodes. An analysis of these characteristics yielded a density of deep-level states of /spl sim/2 /spl times/ 10/sup 17//cm/sup 3/. Deep-level states concentrations were also determined from capacitance-voltage measurements; these were in good agreement with concentrations determined from current-voltage measurements. Recombination lifetimes were extracted from diode reverse recovery storage time measurements.

Blue light emitting diodes (LEDs), based on an AlGaN/GaN/AlGaN double heterojunction structure, have been electrically characterized. These diodes are highly non-ideal with several approximately linear regimes in the semi-logarithmic plots of the forward characteristics. Calculated values of the ideality factor range from 3.0-7.0, indicating that the recombination process in these diodes cannot be conveniently described by the Shockley-Read-Hall statistics. Logarithmic plots of the forward characteristics indicate a space-charge-limited-current (SCLC) conduction through the active region of the diodes. Observed changes in the slope of these logarithmic plots are representative of a high density of "deep-level states" in the active region of the diodes. An analysis of these characteristics yielded an approximate density of deep-level states of 2/spl times/10/sup 17//cm/sup 3/. These states are most likely located close to the valence band edge, in the compensated active region of the diodes.

Blue light emitting diodes (LEDs) based on an AlGaN/GaN/AlGaN double heterojunction structure were electrically characterized as a function of temperature. Current-voltage (I-V), capacitance-voltage (C-V) and reverse recovery storage time measurements were conducted at temperatures in the range between -8° and 75° C. Capacitance-voltage measurements as a function frequency (20 Hz – 1 MHz) and electroluminescence study at room temperature were also performed. It was observed that the diode turn-on voltage decreased with increasing temperature, however, reverse leakage currents monitored at -1, -5 and -10 V showed only a slight increase with increasing temperature. The concentration of deep states and their position in the bandgap, as extracted from logarithmic plots of the forward characteristics, were not influenced by the measurement temperature. Recombination lifetimes, as obtained from experimentally determined reverse recovery storage times, remained constant over the range of temperature considered. A higher value of diode capacitance was observed at low measurement frequencies (20 Hz – 1 kHz), gradually dropping to a lower value over a frequency range between (1 kHz – 100 kHz) and remained constant from 100 kHz to 1 MHz. A loss peak centered about 10 kHz was observed in the corresponding plot of gm/ω as a function of frequency, f. The position of the peak in the gm/ω - frequency (f) plot and dC/dω(for f in the range 1 kHz – 100 kHz ), yielded a concentration of deep-states of approximately 2.2 × 1015/cm3, located at 0.39 eV above the valence bandedge.

A recombination lifetime of approximately 25 ns was extracted from measured reverse recovery storage times in AlGaN/GaN/AlGaN double heterojunction blue light emitting diodes. This experimentally determined lifetime is expected to arise from a combination of radiative and non-radiative processes occurring in the diodes. The non-radiative processes are likely to be due the presence of a high concentration deep-states as identified from the current-voltage and capacitance-voltage measurements. Current-voltage characteristics of these diodes were highly non-ideal as indicated by high values of the ideality factor ranging from 3.0 – 7.0. Logarithmic plots of the forward characteristics indicated a space-charge-limited-current (SCLC) conduction in presence of a high density of “deep-level states” in the active region of the diodes. An analysis of these characteristics yielded an approximate density of these deep-level states as 2 × 1017/cm3. The density of deep-states extracted from capacitance-voltage measurements were in good agreement with that obtained from current-voltage measurements.

Electrical contacts to both n and p-type GaN films have been investigated using electron-beam evaporated and sputtered films of metals such as Al, Au, Cr, Cu, Ni, Pt, and Ti. Films deposited by electron-beam evaporation for the n-type films with doping levels of 1 × 1018/cm3 and lower showed rectifying characteristics with all the metals studied with the exception of Al. Aluminum contact diodes were ohmic in the as-deposited state. The Pt rectifying contact was near-ideal with an ideality factor close to 1.0. Ideality factors for the other metals were much greater than 1. This deviation from thermionic behavior was interpreted as space charge limited current conduction in the presence of deep-level states. Sputtered films showed very similar characteristics to electron-beam deposited films, with the exception of Ti. The Ti contact was ohmic in the as-deposited state. Non-linear Cu contacts to n-type films became ohmic on annealing. However, for p-type films, Ar ion sputter-cleaning prior to metal deposition by sputtering created ohmic contacts with Cu and Pt. Low resistance ohmic contacts were achieved by ion implantation and anneal of Si in n-type and Mg in p-type films, prior to metallization. The implant parameters and anneal temperatures are currently being optimized.