P. Perlin

Investigation of longitudinal‐optical phonon‐plasmon coupled modes in highly conducting bulk GaN

We report a cross‐correlated investigation, performed by means of Raman scattering and infrared spectroscopy, of coupled LO phonon‐plasmon modes in bulk GaN. Using different samples with different (high) residual concentrations of free carriers, we find that the high‐energy Raman mode follows closely the plasma frequency resolved from the infrared data. On the opposite, the low‐frequency modes appears down shifted, with respect to the standard TO phonon frequency, by about 11 cm−1. Both findings agree satisfactorily with predictions of the linear response theory for undamped phonon‐plasmon modes and establish Raman scattering as a powerful and nondestructive tool to investigate the residual doping level of GaN up to about 1020 cm−3 .

Mechanism of yellow luminescence in GaN

We investigated the pressure behavior of yellow luminescence in bulk crystals and epitaxial layers of GaN. This photoluminescence band exhibits a blueshift of 30±2 meV/GPa for pressures up to about 20 GPa. For higher pressure we observe the saturation of the position of this luminescence. Both effects are consistent with the mechanism of yellow luminescence caused by electron recombination between the shallow donor (or conduction band) and a deep gap state of donor or acceptor character.

Thermal expansion of gallium nitride

Lattice constants of gallium nitride (wurzite structure) have been measured at temperatures 294–753 K. The measurements were performed by using x‐ray diffractometry. Two kinds of samples were used: (1) bulk monocrystal grown at pressure of 15 kbar, (2) epitaxial layer grown on a sapphire substrate. The latter had a smaller lattice constant in a direction parallel to the interface plane by about 0.03%. This difference was induced by a higher thermal expansion of the sapphire with respect to the GaN layer. However, this thermal strain was created mainly at temperatures below 500–600 K. Above these temperatures the lattice mismatch in parallel direction diminished to zero at a temperature of about 800 K.

Temperature dependence of the energy gap in GaN bulk single crystals and epitaxial layer

We performed optical‐absorption studies of the energy gap in various GaN samples in the temperature range from 10 up to 600 K. We investigated both bulk single crystals of GaN and an epitaxial layer grown on a sapphire substrate. The observed positions of the absorption edge vary for different samples of GaN (from 3.45 to 3.6 eV at T=20 K). We attribute this effect to different free‐electron concentrations (Burstein–Moss effect) characterizing the employed samples. For the sample for which the Burstein shift is zero (low free‐electron concentration) we could deduce the value of the energy gap as equal to 3.427 eV at 20 K. Samples with a different free‐electron concentration exhibit differences in the temperature dependence of the absorption edge. We explain the origin of these differences by the temperature dependence of the Burstein–Moss effect.

Determination of the effective mass of GaN from infrared reflectivity and Hall effect

Infrared reflectivity and Hall effect measurements were performed on highly conducting n‐type GaN (n≊6×1019 cm−3) bulk crystals grown by the high‐pressure high‐temperature method. Values of electron‐plasma frequency and free‐electron concentration were determined for each sample of the set of seven crystals. It enabled us to calculate the perpendicular effective mass of electrons in the wurtzite structure of GaN as m*=0.22±0.02 m0. Effects of nonparabolicity and a difference between parallel and perpendicular components of the effective mass are small and do not exceed the experimental error.

Degradation mechanisms in InGaN laser diodes grown on bulk GaN crystals

We have investigated the aging processes in InGaN laser diodes fabricated by metal organic vapor phase epitaxy on low-dislocation-density, high-pressure-grown bulk gallium nitride crystals. The measured threshold current turned out to be a square root function of aging time, indicating the importance of diffusion for device degradation. The differential efficiency, in contrast, was roughly constant during these experiments. From these two observations we can conclude that the main reason for degradation is the diffusion-enhanced increase of nonradiative recombination within the active layer of the laser diode. Additionally, microscopic studies of the degraded structures did not reveal any new dislocations within the active area of the aged diodes, thus identifying point defects as a source of nonradiative processes.

Blue-violet InGaN laser diodes grown on bulk GaN substrates by plasma-assisted molecular-beam epitaxy

We report on the InGaN multiquantum laser diodes (LDs) made by rf plasma-assisted molecular beam epitaxy (PAMBE). The laser operation at 408nm is demonstrated at room temperature with pulsed current injections using 50ns pulses at 0.25% duty cycle. The threshold current density and voltage for the LDs with cleaved uncoated mirrors are 12kA∕cm2 (900mA) and 9V, respectively. High output power of 0.83W is obtained during pulse operation at 3.6A and 9.6V bias with the slope efficiency of 0.35W∕A. The laser structures are deposited on the high-pressure-grown low dislocation bulk GaN substrates taking full advantage of the adlayer enhanced lateral diffusion channel for adatoms below the dynamic metallic cover. Our devices compare very favorably to the early laser diodes fabricated using the metalorganic vapor phase epitaxy technique, providing evidence that the relatively low growth temperatures used in this process pose no intrinsic limitations on the quality of the blue optoelectronic components that can be f...

Evidence of free carrier concentration gradient along the c-axis for undoped GaN single crystals

Results of measurements of infrared reflectivity and micro-Raman scattering on the undoped GaN high pressure grown single crystals are reported. These crystals have usually a high electron concentration due to unintentional doping by oxygen. We show, by the shift of the plasma edge (infrared reflectivity measurements), that the free electron concentration is always higher on the (0 0 0 % 1)N face of the GaN single crystal than on the (0 0 0 1)Ga face. In order to determine the profile of the free carrier concentration, we performed transverse micro-Raman scattering measurements along the (0 0 0 1) c-axis of the crystal with spatial resolution of 1mm. Micro-Raman experiments give a quantitative information on the free carrier concentration via the longitudinal optical phonon–plasmon (LPP) coupling modes. Thus, by studying the behavior of the LPP mode along the c-axis, we found the presence of a gradient of free electrons. We suppose that this gradient of electrons is due to the gradient of the main electron donor, in undoped GaN single crystals, i.e. oxygen impurity. We propose a growth model which explains qualitatively the incorporation of oxygen during the growth of GaN crystal under high pressure of nitrogen. # 2001 Elsevier Science B.V. All rights reserved.

Piezoelectric field and its influence on the pressure behavior of the light emission from GaN/AlGaN strained quantum wells

We have studied the influence of hydrostatic pressure on the light emission from a strained GaN/AlGaN multiquantum well system. We have found that the pressure coefficients of the photoluminescence peak energies are dramatically reduced with respect to that of GaN energy gap and this reduction is a function of the quantum well thickness. The decrease of the light emission pressure coefficient may be as large as 30% for a 32 monolayer (8 nm) thick quantum well. We explain this effect by the hydrostatic-pressure-induced increase of the piezoelectric field in quantum structures. Model calculations based on the k×p method and linear elasticity theory reproduce the experimental results well, demonstrating that this increase may be explained by small anisotropy of the wurtzite lattice of GaN and a specific interplay of elastic constants and values of the piezoelectric tensor.

60mW continuous-wave operation of InGaN laser diodes made by plasma-assisted molecular-beam epitaxy

We demonstrate continuous-wave operation at 411nm of InGaN multi-quantum-well laser diodes (LDs) made by plasma-assisted molecular-beam epitaxy (PAMBE). The threshold current density and voltage for these LDs are 4.2kA∕cm2 and 5.3V, respectively. High optical output power of 60mW is achieved. The LDs are fabricated on low-dislocation-density bulk GaN substrates, at growth conditions which resemble liquid-phase epitaxy. We show that use of such substrates eliminates spiral growth, which is the dominant growth mechanism for PAMBE on high-dislocation-density substrates. Therefore, PAMBE opens new perspectives for next generation of InGaN LDs.