H. P. Xin

Nature of room-temperature photoluminescence in ZnO

The temperature dependence of the photoluminescence (PL) transitions associated with various excitons and their phonon replicas in high-purity bulk ZnO has been studied at temperatures from 12 K to above room temperature (320 K). Several strong PL emission lines associated with LO phonon replicas of free and bound excitons are clearly observed. The room temperature PL spectrum is dominated by the phonon replicas of the free exciton transition with the maximum at the first LO phonon replica. The results explain the discrepancy between the transition energy of free exciton determined by reflection measurement and the peak position obtained by the PL measurement.

GaInNAs/GaAs multiple quantum wells grown by gas-source molecular beam epitaxy

GaInNAs/GaAs multiple quantum wells (MQWs) with different N composition were successfully grown on semi-insulating GaAs substrate by gas-source molecular beam epitaxy. A nitrogen radical beam source was used to incorporate N into GaInAs layers. High resolution x-ray rocking curves measurements indicate that the N composition in GaInNAs layer was increased from 0.009 to 0.03 with increasing N2 flow rate. Photoluminescence (PL) measurements show that the PL wavelength red shifts with increasing N composition in GaInNAs layer. For a 7-period Ga0.7In0.3N0.02As0.98/GaAs MQW, a PL peak at 1.3 μm wavelength at room temperature has been successfully obtained. The band offset ΔEc for Ga0.7In0.3NxAs1−x/GaAs enlarges quickly from 0.26 eV to 0.56 eV with increasing N concentration from 0% to 3%.

Self-assembled GaInNAs quantum dots for 1.3 and 1.55 μm emission on GaAs

Ga0.3In0.7NxAs1−x (x⩽0.04) and InNxAs1−x (x⩽0.02) self-assembled quantum dots are fabricated on GaAs by gas-source molecular beam epitaxy. The effect of growth temperature on island density, island size, and optical properties is studied in detail. From a single layer of Ga0.3In0.7N0.04As0.96 dots embedded in GaAs, peak photoluminescence wavelength of up to 1.52 μm is detected at room temperature. Thus, the fabrication of 1.3 and 1.55 μm GaInNAs quantum dot lasers on GaAs becomes feasible.Ga0.3In0.7NxAs1−x (x⩽0.04) and InNxAs1−x (x⩽0.02) self-assembled quantum dots are fabricated on GaAs by gas-source molecular beam epitaxy. The effect of growth temperature on island density, island size, and optical properties is studied in detail. From a single layer of Ga0.3In0.7N0.04As0.96 dots embedded in GaAs, peak photoluminescence wavelength of up to 1.52 μm is detected at room temperature. Thus, the fabrication of 1.3 and 1.55 μm GaInNAs quantum dot lasers on GaAs becomes feasible.

Nature of the fundamental band gap in GaNxP1−x alloys

The optical properties of GaNxP1−x alloys (0.007⩽x⩽0.031) grown by gas-source molecular-beam epitaxy have been studied. An absorption edge appears in GaNxP1−x at energy below the indirect ΓV–XC transition in GaP, and the absorption edge shifts to lower energy with increasing N concentration. Strong photomodulation signals associated with the absorption edges in GaNxP1−x indicate that a direct fundamental optical transition is taking place, revealing that the fundamental band gap has changed from indirect to direct. This N-induced transformation from indirect to direct band gap is explained in terms of an interaction between the highly localized nitrogen states and the extended states at the Γ conduction-band minimum.

Observation of quantum dot-like behavior of GaInNAs in GaInNAs/GaAs quantum wells

We report a quantum dot-like behavior of GaInNAs due to composition nonuniformity of N and In in GaInNAs/GaAs quantum wells (QWs). Images of cross-sectional transmission electron microscopy show that the wells of both Ga0.7In0.3As/GaAs and Ga0.7In0.3N0.02As0.98/GaAs are undulated due to lateral variations in strain. This effect is more pronounced in the N-containing QWs due to nonuniform In and N concentrations. Rapid thermal annealing causes a blueshift of the photoluminescence (PL) peak, and results in a splitting of the as-grown broad PL emission into two peaks. The In and N composition fluctuation after annealing becomes predominantly bimodal. The low-energy PL peak is attributed to excitons localized at deep levels, which are originated from In- and N-rich regions in the wells acting as quantum dots (QD). The high-energy peak PH is likely due to the excitons of the 2D QWs. To reduce the local strain, N atoms are preferentially localized in the In-rich regions, so the separation between these two peak...

Effects of nitrogen on the band structure of GaNxP1−x alloys

We report that the incorporation of N in GaNxP1−x alloys (x⩾0.43%) leads to a direct band-gap behavior of GaNP. For N concentration lower than 0.43%, a series of sharp emission lines from the various N pair centers are observed for GaNP bulk layers. With increasing N concentration higher than 0.43%, a strong photoluminescence (PL) emission from GaNP bulk layers is observed at room temperature. While the PL peak redshifts with increasing N concentration to 3.1%, the PL intensity remains as intense. Absorption measurements show a direct band-gap behavior of GaNP alloys.

Band anticrossing in III-N-V alloys

Recent high hydrostatic pressure experiments have shown that incorporation of small amounts of nitrogen into conventional III–V compounds to form III–N–V alloys leads to splitting of the conduction band into two subbands. The downward shift of the lower subband edge is responsible for the observed, large reduction of the fundamental band gaps in III–N–V alloys. The observed effects were explained by an anticrossing interaction between the conduction band states close to the center of the Brillouin zone and localized nitrogen states. The interaction leads to a change in the nature of the fundamental from the indirect gap in GaP to a direct gap in GaNP. The predictions of the band anticrossing model of enlarged electron effective mass and enhanced donor activation efficiency were confirmed by experiments in GaInNAs alloys.

Annealing behavior of p-type Ga0.892In0.108NxAs1−x(0⩽X⩽0.024) grown by gas-source molecular beam epitaxy

P-type, Be-doped GaInNAs layers (1100 A thick) are grown on GaAs substrates by gas-source molecular beam epitaxy with a nitrogen radical beam source. High-resolution x-ray rocking curves show that the Ga0.892In0.108NxAs1−x peak shifts closer to the GaAs substrate peak with increasing N concentration, indicating reduced strain. After rapid thermal annealing (RTA) at 700 °C for 10 s, the Ga0.892In0.108As sample suffers strain relaxation, but the N-containing samples remain pseudomorphically strained, suggesting better thermal stability of GaInNAs. The wavelength of room-temperature photoluminescence redshifts from 0.988 to 1.276 μm, due to large band gap bowing, with N concentration increased from 0 to 0.024. Secondary ion mass spectrometry results show no Be diffusion, but hydrogen incorporation alongside N. The free carrier concentration is decreased by one order of magnitude mainly due to H passivation, but after RTA at 700 °C, it is increased to half that of GaInAs due to the reduced H concentration. Th...

Heterojunction bipolar transistors implemented with GaInNAs materials

Use of GaInNAs in the base of heterojunction bipolar transistors (HBTs) on GaAs substrates allows a reduction of the turn-on voltage, Vbe,on, of the devices, facilitating their use in applications with low power supply voltage (particularly battery operated power amplifiers for mobile communications). Using GaInNAs with N content below 2% and In content of 1–20%, HBTs have been demonstrated with Vbe,on values lower by 25–400 mV than those of conventional GaAs-based HBTs. The GaInNAs base regions exhibit lower diffusion length than conventional GaAs bases, which reduces current gain and detracts from high-frequency performance, as well as higher base sheet resistance. These adverse effects can be mitigated by proper design tradeoffs of base thickness and nitrogen composition, as well as by compositional grading in the base to provide a built-in quasi-electric field to assist electron transport.

GaN0.011P0.989 red light-emitting diodes directly grown on GaP substrates

Red light-emitting diodes (LEDs) emitting at 670 nm and employing GaN0.011P0.989 p–n homojunction grown on a (100) GaP substrate by gas-source molecular beam epitaxy with a rf plasma nitrogen source have been obtained. The integrated photoluminescence intensity of GaNP p–n homojunction LED is 5 times stronger than that of Ga0.51In0.49P bulk layer, but the peak width is much broader. Compared to conventional high-brightness AlGaInP red LEDs, our LED structure saves two process steps of etch removing of the GaAs absorbing substrate and wafer bonding to a GaP transparent substrate.