M. S. Minsky

Effective band gap inhomogeneity and piezoelectric field in InGaN/GaN multiquantum well structures

The emission mechanisms of strained InxGa1−xN quantum wells (QWs) were shown to vary depending on the well thickness, L, and x. The absorption edge was modulated by the quantum confined Stark effect and quantum confined Franz-Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of the piezoelectric field, FPZ, and L exceed the valence band discontinuity, ΔEV. In this case, holes are confined in the triangular potential well formed at one side of the well producing the apparent Stokes-like shift. Under the condition that FPZ×L exceeds the conduction band discontinuity ΔEC, the electron-hole pair is confined at opposite sides of the well. The QCFK further modulated the emission energy for the wells with L greater than the three dimensional free exciton Bohr radius aB. On the other hand, effective in-plane localization of carriers in quantum disk size potential minima, which are produced by nonrandom alloy compositional fluctuation enhanced by the large bowing parameter and...

Room‐temperature photoenhanced wet etching of GaN

Laser‐enhanced, room‐temperature wet etching of GaN using either dilute HCl:H2O (1:10) or 45% KOH:H2O(1:3) is reported. Etch rates of a few hundred A/min (HCl) and up to a few thousand A/min (KOH) have been measured for unintentionally doped n‐type films of thickness (1–2 μm) grown by MOCVD on a sapphire substrate. The etching is thought to take place photoelectrochemically with holes and electrons generated by incident illumination from 4.5 mW of HeCd laser power enhancing the oxidation and reduction reactions in an electrochemical cell.

Emission mechanisms of bulk GaN and InGaN quantum wells prepared by lateral epitaxial overgrowth

The emission mechanisms of bulk GaN and InGaN quantum wells (QWs) were studied by comparing their optical properties as a function of threading dislocation (TD) density, which was controlled by lateral epitaxial overgrowth. Slightly improved excitonic photoluminescence (PL) intensity was recognized by reducing TD density from 1010 cm−2 to less than 106 cm−2. However, the major PL decay time was independent of the TD density, but was rather sensitive to the interface quality or material purity. These results suggest that TDs simply reduce the net volume of light-emitting area. This effect is less pronounced in InGaN QWs where carriers are effectively localized at certain quantum disk size potential minima to form quantized excitons before being trapped in nonradiative pathways, resulting in a slow decay time. The absence of any change in the optical properties due to reduction of TD density suggested that the effective band gap fluctuation in InGaN QWs is not related to TDs.

Influence of Si doping on characteristics of InGaN/GaN multiple quantum wells

We have systematically studied the influence of Si doping on the characteristics of InGaN/GaN multiple quantum wells (MQWs) by means of high-resolution x-ray diffraction (HRXRD), photoluminescence (PL), PL excitation (PLE), and time-resolved PL spectroscopy. The twelve- period MQWs were grown by metalorganic chemical vapor deposition. Si doping in the GaN barriers was varied from 1×1017 to 3×1019 cm−3. Information on the structural quality of the MQWs as a function of Si doping was extracted from the linewidth broadening of the higher-order superlattice satellite peaks measured in HRXRD. The HRXRD measurements indicate that increased Si doping results in better interface properties of the MQWs. PL and PLE measurements show a decrease in the Stokes shift with increasing Si doping concentration. The 10 K radiative recombination lifetime was observed to decrease from ∼30 ns (for n<1×1017 cm−3) to ∼4 ns (for n=3×1019 cm−3) with increasing Si doping concentration. The reduced Stokes shift, the decrease in radi...

CHARACTERIZATION OF HIGH-QUALITY INGAN/GAN MULTIQUANTUM WELLS WITH TIME-RESOLVED PHOTOLUMINESCENCE

Recombination in single quantum well and multiquantum well InGaN/GaN structures is studied using time-resolved photoluminescence and pulsed photoluminescence measurements. Room-temperature measurements show a rapid lifetime (0.06 ns) for a single quantum well structure, while an increasingly long decay lifetime is measured for multiquantum wells as more quantum wells are incorporated into the structure. Temperature-dependent lifetime measurements show that a nonradiative recombination mechanism activates above 45 K in the single quantum well but is less important in the multiquantum wells.

EFFECTS OF SI-DOPING IN THE BARRIERS OF INGAN MULTIQUANTUM WELL PURPLISH-BLUE LASER DIODES

Optical gain spectra of InGaN multiquantum well laser diode wafers having Si-doped or undoped InGaN barriers were compared. Although evidence for effective band-gap inhomogeneity was found in both structures, the wells with the Si-doped barriers exhibited a smaller Stokes-like shift. Si doping suppressed emergence of a secondary amplified spontaneous emission peak at 3.05 eV, which was uncoupled with the primary one at 2.93 eV. Furthermore Si doping reduced the threshold power density required to obtain the stimulated emission.

Radiative recombination lifetime measurements of InGaN single quantum well

We present results from a time‐resolved study of radiative recombination in InGaN quantum wells. The sample was grown by atmospheric pressure metal‐organic chemical‐vapor deposition. Time‐resolved photoluminescence measurements were performed from 7 K up to room temperature. The low temperature radiative lifetime was measured to be on the order of 250 ps at a generated carrier density of 1012 cm−2. The time‐resolved measurements show a bimolecular recombination characteristic. At 300 K, we observed a lifetime of 130 ps which, to the best of our knowledge, is the longest lifetime reported for any III–V nitride at room temperature.

Time-resolved photoluminescence studies of InGaN/GaN single-quantum-wells at room temperature

We present a room-temperature study of the well-width-dependent carrier lifetimes in InGaN single-quantum wells. At room temperature, carrier recombination was found to be dominated by interface-related nonradiative processes. The dominant radiative recombination at room temperature was through band-to-band free carriers. For the sample grown at a higher growth rate, we observed a longer luminescence lifetime, which was attributed to an improved quantum well interface.

Growth and properties of InGaN nanoscale islands on GaN

Strong photoluminescence and radiative recombination lifetimes longer than 1 ns at room temperature have been observed in GaN/Si/InGaN/GaN structures containing InGaN submicron islands. The flat islands, with a width at their base in the order of 200 nm and a height in the order of 1—2 nm, grow in a spiral mode around dislocations with partial or pure screw character after a passivation of the GaN surface by a preflow of disilane. Their surface density is comparable to the dislocation density of the GaN layer in the order of 108—109 cm~2. ( 1998 Elsevier Science B.V. All rights reserved.

Optical Properties of InGaN/GaN Quantum Wells with Si Doped Barriers

Optical properties of InGaN/GaN multi-quantum wells with Si doped barriers are studied using cw and time resolved photoluminescence (TRPL) and photoluminescence excitation (PLE) spectroscopy. The room temperature carrier lifetime depends strongly on the Si doping level in the quantum well barriers, decreasing from 10 ns to 1 ns as the doping level is increased from unintentionally doped to 5×1018 cm-3 (Si:GaN). The shift between the absorption edge and emission peak decreases from 220 meV to 110 meV as the doping is increased. Temperature dependent photoluminescence measurements indicate a higher density of non-radiative centers in the undoped structures.