B. Goldenberg

Temperature dependence of interband transitions in GaN grown by metalorganic chemical vapor deposition

The interband transitions in single‐crystal GaN films grown by metalorganic chemical vapor deposition (MOCVD) have been studied as a function of temperature (15≤T≤300 K) by reflectance and photoluminescence measurements. At low temperatures, well‐resolved spectral features corresponding to the GaN band structure were observed. The energies of the excitonic interband ΓV9−ΓC7,ΓV7 (upper band)−ΓC7 and ΓV7(lower band)−ΓC7 transitions are found to be 3.485, 3.493, and 3.518 eV at 15 K, respectively, for the MOCVD GaN. The spectral features are broadened and shift to lower energy as temperature increases. At room temperature (300 K), the ΓV9−ΓC7and ΓV7 (upper band) −ΓC7 transition energies of this wide band‐gap material are determined to be 3.420 and 3.428 eV, respectively. The temperature dependence of these two transitions have been determined using the Varshni empirical relation. Our results yield E0(T)=3.486–8.32×10−4 T2/(835.6+T) eV for the ΓV9−ΓC7 transition and E0(T)=3.494–10.9×10−4 T2/(1194.6+T) eV for ...

High speed, low noise ultraviolet photodetectors based on GaN p-i-n and AlGaN(p)-GaN(i)-GaN(n)structures

We have investigated the spectral response of front-surface-illuminated GaN and AlGaN/GaN p-i-n ultraviolet photodetectors prepared by reactive molecular beam epitaxy on sapphire substrates. GaN homojunction p-i-n photodiodes exhibited a peaked response near the band edge. This enhanced response was absent in the AlGaN/GaN heterojunction p-i-n detectors. We analyzed the effect of p-layer thickness of the GaN p-i-n diodes on the magnitude of the peak photoresponse. The AlGaN/GaN photodiodes had a maximum zero-bias responsivity of 0.12 A/W at 364 nm, which decreased by more than 3 orders of magnitude for wavelengths longer than 390 nm. A reverse bias of −10 V raised the responsivity to 0.15 A/W without any significant increase in noise. The root-mean-square noise current in a 1 Hz bandwidth is ∼1.0 pA, corresponding to a noise-equivalent-power of ∼8.3 pW. We measured extremely fast decay times of 12 ns for the AlGaN/GaN and 29 ns for the GaN photodiodes.

Defeating Compensation in Wide Gap Semiconductors by Growing in H that is Removed by Low Temperature De-Ionizing Radiation

We propose a general method to obtain high conductivity of either type in wide gap semiconductors where compensation normally limits conductivity of one or both types. We suggest that the successes of Amano et al. and of Nakamura et al. in obtaining more than 1018 cm-3 holes in GaN are particular examples of the general process that we propose.

Above room temperature near ultraviolet lasing from an optically pumped GaN film grown on sapphire

Optically pumped near ultraviolet lasing from single‐crystal GaN grown by metalorganic chemical vapor deposition has been achieved over a temperature range from 10 K to over 375 K by using a side‐pumping geometry on small barlike samples. The laser emission threshold was measured as a function of temperature and the threshold was found to show weak temperature dependence: ∼500 kW/cm2 at 10 K and ∼800 kW/cm2 at room temperature (295 K) for one particular sample studied. The longitudinal lasing modes were clearly observed. The characteristics of the temperature dependence of the laser emission threshold suggests that GaN is a suitable material for the development of optoelectronic devices required to operate at high temperatures.

Ultraviolet and violet light‐emitting GaN diodes grown by low‐pressure metalorganic chemical vapor deposition

Both metal‐insulator‐semiconductor and p‐n junction electroluminescence have been observed in thin‐film, metalorganic chemical vapor deposition‐grown GaN diodes thermally annealed in N2. UV radiation, peaking near 380 nm, is emitted when electrons are injected from the undoped, n‐type material into the Mg‐doped, p‐type GaN. Violet light, peaking near 430 nm, is obtained by injecting electrons into p‐type material from either n‐type material or non‐ohmic metal contacts. The present results support and extend earlier interpretations of the nature of the recombination centers in GaN.

Mechanisms of band‐edge emission in Mg‐doped p‐type GaN

Time‐resolved photoluminescence has been employed to study the mechanisms of band‐edge emissions in Mg‐doped p‐type GaN. Two emission lines at about 290 and 550 meV below the band gap (Eg) have been observed. Their recombination lifetimes, dependencies on excitation intensity, and decay kinetics have demonstrated that the line at 290 meV below Eg is due to the conduction band‐to‐impurity transition involving shallow Mg impurities, while the line at 550 meV below Eg is due to the conduction band‐to‐impurity transition involving doping related deep‐level centers (or complexes).

Time‐resolved exciton luminescence in GaN grown by metalorganic chemical vapor deposition

We report the results of time‐resolved studies on the exciton radiative decay in single‐crystal GaN films grown by metalorganic chemical vapor deposition. Time‐resolved photoluminescence (PL) measurements were performed on the samples at various temperatures from 10 to 320 K. The well‐resolved near‐band‐edge luminescence features associated with free excitons and bound excitons in the GaN allow us to unambiguously determine their decay times. We found that the nonradiative recombination processes play an important role and dominate the decay of exciton population. The processes depend on the density of defects and impurities in the GaN samples.

Temperature-dependent absorption measurements of excitons in GaN epilayers

Optical absorption measurements were performed on a series of thin GaN epilayers. Sharp spectral features were observed due to the 1s A and B exciton transitions. Using polarization dependent absorption, the C exciton transition was identified. A broad absorption feature was observed at ∼3.6 eV, which is attributed to indirect exciton-phonon absorption. The excitonic structure was found to persist well above room temperature. A fit to the Varshni formula yielded a temperature dependence of E(T)=E(T=0)−11.8×10−4T2(1414+T) eV for the A and B excitons. The exciton absorption linewidth was studied as a function of temperature, indicating that GaN exhibits very large exciton-phonon coupling.

PRESSURE-DEPENDENT PHOTOLUMINESCENCE STUDY OF WURTZITE GAN

Low‐temperature photoluminescence (PL) in single‐crystal GaN films grown on sapphire substrates by metalorganic chemical vapor deposition has been studied as a function of applied hydrostatic pressure using the diamond‐anvil‐cell technique. The PL spectra of the GaN at atmospheric pressure were dominated by two sharp, strong, near‐band‐edge exciton luminescence lines and a broad emission band in the yellow spectral region. The exciton emission lines were found to shift almost linearly toward higher energy with increasing pressure. While the yellow emission band showed a similar blue shift behavior under applied pressure, a relatively strong sublinear pressure dependence was observed. By examining the pressure dependence of the exciton emission structures, the pressure coefficient of the direct Γ band gap in the wurtzite GaN was determined. The value of the hydrostatic deformation potential of the band gap has also been deduced from the experimental results.

Optical properties of wurtzite GaN grown by low‐pressure metalorganic chemical‐vapor deposition

We present the results of optical studies on the properties of GaN grown by low‐pressure metalorganic chemical‐vapor deposition, with emphasis on the issues vital to device applications such as stimulated emission and laser action as well as carrier relaxation dynamics. By optical pumping, stimulated emission and lasing were investigated over a wide temperature range up to 420 K. Using a picosecond streak camera, the free and bound exciton emission decay times were examined. In addition, the effects of temperature and pressure on the optical interband transitions and the transitions associated with impurity/defect states were studied using a variety of spectroscopic methods, including photoluminescence and photoreflectance. The fundamental band gap of GaN was mapped out as a function of temperature using the empirical Varshni relation. The pressure coefficient of the gap was determined using diamond‐anvil pressure‐cell technique. The hydrostatic deformation potential for the direct Γ band gap was also der...