S. Krishnankutty

Growth of high optical and electrical quality GaN layers using low‐pressure metalorganic chemical vapor deposition

We report on the low‐pressure metalorganic chemical vapor deposition of high quality single‐crystal GaN layers over basal plane sapphire substrates. Optimization of growth conditions resulted in material with carrier densities of 1017 /cm3 at room temperature and corresponding mobilities around 350 cm2 /V s. The photoluminescence linewidths improved from 160 meV [full width at half maximum (FWHM)] to 25 meV (FWHM). With improved material quality we were able to observe the polar optical mode and the ionized impurity scattering regimes in the mobility versus temperature data. Good quality Schottky barriers were formed on the as‐grown material using a tungsten probe and an alloyed indium contact. Our observations indicate a direct correlation between electrical and optical characteristics of good material and strongly question nitrogen vacancies as the sole explanation for the high carrier densities observed in poor quality GaN growths.

Dominance of tunneling current and band filling in InGaN/AlGaN double heterostructure blue light‐emitting diodes

Measurement of the room temperature forward bias current‐voltage behavior of InGaN/AlGaN double heterostructure blue light‐emitting diodes demonstrates a significant departure from the usual Is exp(qV/ nkT) behavior where n is the ideality factor which varies between 1 and 2. The observed current‐voltage behavior at room temperature may be represented as I=2.7×10−11 exp(5.7V) which suggests a tunneling mechanism. Measurement of the electroluminescence for currents from 0.5 to 100 mA demonstrates that the emission peak shifts to higher energy while increasing in intensity. The shifting peak spectra is due to band filling, a process which results from the injection of holes via tunneling into an empty acceptor impurity band and vacant valence band tails. At currents near 100 mA, a non‐shifting band‐to‐band emission approaches the intensity of the shifting peak spectra. The active layer of these diodes is codoped with both the donor Si and the acceptor Zn.

Photoluminescence characteristics of AlGaN‐GaN‐AlGaN quantum wells

AlxGa1−xN‐GaN quantum wells were grown on basal plane sapphire by low‐pressure metalorganic vapor deposition. The photoluminescence spectra of samples of different well thicknesses and x values were measured. The experimental data were compared with the calculated solutions of the finite square quantum well and the bound states involved in the optical transition were identified.

Vertical–cavity stimulated emission from photopumped InGaN/GaN heterojunctions at room temperature

We report the observation of room temperature violet (415 nm) stimulated emission in the vertical cavity mode from photopumped GaN/In0.25Ga0.75N heterojunctions. The InGaN/GaN heterojunction was deposited over sapphire substrates using low‐pressure metalorganic chemical vapor deposition and was of high enough optical quality to achieve room‐temperature stimulated emission. The observed emission intensity was found to be a nonlinear function of incident optical pump power density. At threshold we observe a clear line narrowing of the output optical signal from 20 to 1.5 nm full width at half‐maximum.

Low pressure metalorganic chemical‐vapor deposition of cubic GaN over (100) GaAs substrates

We report on the low pressure metal organic chemical‐vapor deposition of single crystal cubic GaN films over (100) GaAs substrates. Using photoluminescence and direct optical absorption measurements we estimate the band gap for c‐GaN at room temperature to be 3.3 eV. Reflection high energy electron diffraction, x‐ray, transmission electron microscopy, optical absorption, and room‐temperature photoluminescence data are presented to establish the quality of a 0.8‐μm‐thick cubic GaN film over (100) GaAs substrate. Preliminary measurement results for the carrier density and mobility of the as‐deposited c‐GaN film are also presented.

Photoluminescence characterization of AlGaN-GaN pseudomorphic quantum wells and calculation of strain induced bandgap shifts

The low temperature (77 K) photoluminescence characteristics of AlxGa1-xN-GaN strained layer quantum wells with differentx values grown by metalorganic chemical vapor deposition (MOCVD) were investigated. The photoluminescence spectra were useful in analyzing both quantum confinement effects and strain induced energy shifts. The strain induced shifts were found to be a strong function of aluminum compositionx. A model was developed to calculate the strain induced bandgap shifts atk = 0. The values predicted by this model which took into account the wurtzite crystal structure of the material system, were in good agreement with (i.e. within 2 meV of) the experimentally measured shifts.

Effect of shroud flow on high quality In x Ga 1−x N deposition in a production scale multi-wafer-rotating-disc reactor

High quality InGaN thin films and InGaN/GaN double heterojunction (DH) structures have been epitaxially grown on c-sapphire substrates by MOCVD in a production scale multi-wafer-rotating-disc reactor between 770 to 840°C. We observed that shroud flow (majority carrier gas in the reaction chamber) is the key to obtaining high quality InGaN thin films. High purity H2 as the shroud flow results in poor crystal quality and surface morphology but strong photolumines-cence (PL) at room temperature. However, pure N2 as the shroud flow results in high crystal quality InGaN with an x-ray full width at half maximum (FWHM)InGaN(0002) of 7.5 min and a strong room temperature PL peaking at 400 nm. In addition, InGaN/GaN single heterojunction (SH) and DH structures both have excellent surface morphology and sharp interfaces. The full width at half maximum of PL at 300K from an InGaN/GaN DH structure is about 100 meV which is the best reported to date. A high indium mole fraction in InGaN of 60% and high quality zinc doped InGaN depositions were also achieved.

Optical characterization of AlGaN-GaN-AlGaN quantum wells

High quality ALxGa1−xN-GaN-AlxGa1−xN quantum wells of different thicknesses andx values were grown by low pressure metalorganic chemical vapor deposition (LPMOCVD). The change in their emission energies (measured at 77 K by photoluminescence) as a function of both well width andx value was typical of a type I heterojunction. The experimental data was compared to theoretical calculations based on the finite square well model and the confined particle transitions were identified. The experimentally observed energy shifts differed from calculated values of then = 1 electron to heavy hole transition by a constant amount (for a givenx value) attributed to strain in the AlGaN-GaN system. Also, an estimate of the critical thickness in the AlGaN-GaN system was determined based on the Matthews and Blakeslee force balance model.

Cathodoluminescence of AlN–GaN short period superlattices

Cathodoluminescence of AlN–GaN short period superlattice films was measured at 6 K, 77 K, and room temperature. The superlattice films were deposited using a switched atomic layer metalorganic chemical vapor deposition process onto a buffer layer of either AlN or GaN, which was deposited on basal plane sapphire substrates. The individual AlN and GaN layers of the superlattice films ranged in thickness from 2.6 to 20.8 A. The cathodoluminescence of these samples was measured at several electron acceleration voltages to allow depth profiling of the samples. This allows the region of the sample (superlattice film, buffer layer, and substrate) from which the spectral features originate to be determined. A spectral peak in the ultraviolet region above the 3.5 eV band gap of GaN has been observed in all the superlattice samples studied to date. Our results indicate that the location of this peak is determined by quantum confinement in the GaN layers.

Photoconductive and photovoltaic ultraviolet sensors based on GaN

We have fabricated photoconductive and photovoltaic ultraviolet sensors from GaN single layers and pn-junctions. These sensors exhibit a sharp long wavelength cut-off in responsivity at the bandgap (365 nm). The active layers (GaN) were deposited using low pressure MOCVD. The p-type doping was accomplished using Mg as the dopant. Photoconductive, and schottky barrier detectors were then fabricated using photolithography, reactive ion etching and contact metallizations. These processing techniques were developed specific to the A1xGa1-xN material system. We will discuss growth, fabrication and characterization details for these various device types. The measured values of device parameters will be contrasted with those estimated from active layer material characterization.