V. Härle

Interface characteristics of GaInP/GaAs double heterostructures grown by metalorganic vapor phase epitaxy

Abstract The formation of parasitic Ga x In 1− x As y P 1− y intermediate layers when growing GaAs on GaInP or vice versa by MOVPE is investigated by the use of asymmetric GaInP/GaAs/AlGaAs double heterostructures. We found that such intermediate layers are only formed when growing GaAs on GaInP (inverted interface), but not for the reverse case (normal interface), resulting in a strong photoluminescence peak below the GaAs band gap. Time-resolved photoluminescence also indicates a much shorter minority carrier lifetime of the structures containing the inverted GaInP/GaAs interface (∼10 ns) compared to samples containing the normal interface (∼0.5 μs). By investigation of the interface of GaInP/AlGaAs and AlGaInP/GaAs, it is found that In carry-over is mainly responsible for the formation of this layer in our system. By growing thin AlGaAs intermediate layers (1 to 2 nm) at the inverted interface, this low-energy photoluminescence peak could be effectively suppressed. The same could be obtained without any special means in a smaller epitaxial system, probably due to the reduced hot susceptor surface.

Low pressure MOVPE of GaN and GaInNGaN heterostructures

Abstract GaN single layers and GaInN GaN heterostructures have been grown by low pressure metalorganic vapor phase epitaxy on sapphire substrates. We found best growth conditions and the highest growth rate for GaN to be at about 1000°C, whereas the growth rate decreased for both, higher and lower temperatures. In contrast, GaInN with a significantly high In content could only be grown at lower temperatures around 700°C. Besides growth temperature and reactor pressure, the composition of the carrier gas was found to play an important role: the In incorporation rate is about doubled when reducing the hydrogen/nitrogen ratio. GaInN GaN quantum wells show even higher In contents compared to bulk layers.

Phonons and free carriers in strained hexagonal GaN/AlGaN superlattices measured by infrared ellipsometry and Raman spectroscopy

Abstract Phonon and free-carrier effects in strained hexagonal (α) {GaN}l−{AlxGa1−xN}m superlattice (SL) heterostructures are studied by infrared spectroscopic ellipsometry (IRSE) and micro (μ)-Raman scattering. Growth of the heterostructures was performed by metal–organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy on (0xa00xa00xa01) sapphire. Unstrained 0.5–1 μm-thick α-GaN layers were deposited prior to the SLs. SL phonon modes are identified combining results from both IRSE and μ-Raman techniques. The average compressive SL stress can be estimated from the shift of the GaN-sublayer phonon modes. The IRSE data are sensitive to free carriers within the GaN sublayers. For the MOVPE grown SL structures, the free-carrier mobility is anisotropic which indicates vertical carrier confinement.

Relaxation of thermal strain in GaN epitaxial layers grown on sapphire

Abstract We have investigated GaN layers of various thicknesses grown on (0001) c-face sapphire by metalorganic vapor phase epitaxy (MOVPE) with an intermediate 10 nm AlN nucleation layer, in order to study the thermal strain in these layers. Using spatially resolved cathodoluminescence (CL) spectroscopy at low temperatures we investigated the relaxation of this stress at the cleaved edges of the samples as can be observed in an energy shift of the luminescence peak. The shift is compared with a theoretical model for the thermal stress in a rectangular plate clamped along one edge. It was found that the sapphire is also strained, but in a tensile way. An effective deformation potential of 12 eV, as found by Amano et al. [6] was confirmed by direct measurement of the energy shift and supplementing X-ray diffractometry. The difference in thermal expansion coefficients between GaN and sapphire perpendicular to the c-face for temperatures of 6-300 K was estimated to 1.2 × 10 −6 K −1 . Furthermore the freeze-in temperatures for thermal induced dislocation enhancement is estimated.

Mechanisms of Strain Reduction in GaN and AlGaN/GaN Epitaxial Layers

We have investigated Al 0.12 Ga 0.88 N layers with and without a 1 µ m GaN buffer, grown on the c-face of α – Al 2 O 3 substrate with an intermediate AlN nucleation layer grown by LP-MOVPE. We used spatially resolved cathodoluminescence spectroscopy at a temperature of 8K to investigate the strain and the homogeneity of composition that can be determined from the energy of the luminescence peak. The larger thermal expansion coefficient of the sapphire in comparison to the nitrides leads to a biaxial compressive strain of the upper GaN layer when cooling down from growth temperature. For AlGaN layers directly grown on the nucleation layer this cannot be confirmed. The layer stays relaxed and fluctuations in the aluminium composition of 0.4% can be observed. When growing an intermediate GaN buffer, the AlGaN layer gets tensilely strained. This strain is of elastic nature and microcracks can be observed preferentially at the edges due to the smaller lattice constant of AlGaN in comparison to GaN. Even detaching of the AlGaN layers grown on the buffer can be observed. In the regions without cracks the layers are quite homogeneous. A deformation potential of (19±4)eV was estimated for Al 0.12 Ga 0.88 N.

Near-bandgap photoluminescence decay time in GaN epitaxial layers grown on sapphire

Using picosecond time-resolved photoluminescence the authors have studied the decay time of excess carriers in GaN epitaxial layers over a wide range of temperatures from 4 K up to 400 K. At low temperature, a thermal dissociation of donor-bound excitons is observed. At higher temperatures up to room temperature, the luminescence decay at moderate excitation is governed by trapping of photogenerated electrons in ionized shallow donor levels. Using measured luminescence intensities to determine the quantum efficiency, the authors obtain the radiative lifetime of free excitons from low temperature up to room temperature. They use these data to determine the radiative recombination coefficient and the interband momentum matrix element.

Quantum confined Stark effect in InGaAs/InP and InGaAs/InGaAsP multi quantum well structures

Abstract Differential electrotransmission measurements on InGaAs/InP and InGaAs/InGaAsP multi quantum well structures (MQW) with well widths 8,10 and 20 nm with InGaAsP barriers of E g (77 K ) = 1.2 eV are presented. The differential electrotransmission spectra were fitted with a lorentzian lineshape model for the excitonic transition. A Stark shift of up to 18 meV was observed for the 8 nm InGaAs/InGaAsP quantum well structure. The results were compared to a calculation of the Stark effect including the field dependent exciton binding energy. We find good agreement between experiment and theory. The well width dependence of the oscillator strength shows a maximum for L z = 11 nm when changing the field from 1 · 10 4 V/cm to 1 · 10 5 V/cm .

Electric field effects on excitons in gallium nitride

Abstract Electric field effects on Wannier excitons are observed for the first time in GaN thin films. Using both absorption and photocurrent measurements, we have studied the excitonic Franz-Keldysh effect in thin epitaxial GaN films at temperatures between 80 and 300 K. We have measured the Stark shift, quenching and broadening of the exciton peak with applied field. These results are compared with theoretical calculations from the literature. The physics of exciton ionization at varying temperatures is discussed which explains the interplay between absorption and photocurrent. An UV modulator based on the excitonic Franz-Keldysh effect is demonstrated.

Optically detected spin resonance of conduction band electrons in InGaAs/InP quantum wells

Optically detected spin resonance was used to measure the effective g-value of electrons at the conduction band minimum in type-I quantum wells. The experiments showed that the spin resonance is induced by electric dipole transitions, and hence is not limited by the short carrier lifetime that renders magnetic dipole transitions impossible. The spin splittings obtained are strongly anisotropic and dependent on quantum well thickness. A calculation without adjustable parameters, using a three-band Kane model, agrees with the experimental data. The bulk effective g-value of used in this calculation was measured on a thick sample.

Optical gain in the nitrides: are there differences to other III–V semiconductors?

Abstract We have studied the optical gain spectra in GalnN/GaN and GaN/AlGaN double heterostructures and quantum wells at room temperature employing the stripe-excitation method. We compare the results with data for other III–V semiconductors. The optical gain is strongly anisotropic, with almost no gain for the TM mode. For quantum wells, the material gain increases with decreasing well width as expected. Whereas for GalnN/GaN structures only a single gain peak is observed, consistent with a free-carrier gain model, measurements on GaN/AlGaN structures reveal two peaks, which are assigned to localized exciton and exciton-LO-phonon gain.