Semiconductor lasers have a wide range of applications in industry and our daily life. Vertical-Cavity Surface-Emitting Lasers (VCSELs), developed from the 1980’s, have several advantages, compared to edge-emitting lasers, such as wafer-scale testing, uniformity, scalability, circular beam, single-mode, 2D array, etc. A VCSEL consists of a light-emitting active region sandwiched between a top Distributed Bragg Reflector (DBR) and a bottom DBR. In 2017, Apple released the iPhone X embedding VCSELs for face recognition. This was a game-changer. The market is expected to reach $1.9 billion in 2027. Current VCSELs on cell phones for facial recognition are based on 940 nm VCSELs consisting of GaAs/AlAs DBRs grown on GaAs(001) substrates. It is desirable to have longer-wavelength VCSELs, however, because the screen of a smart phone is transparent at longer wavelength (1380 nm) and because of eye safety. The maximum permissible exposure to the retina is higher for wavelength longer than 940 nm. Long-wavelength lasers beyond 1300 nm is commonly fabricated on InP substrates because those wavelengths cannot be accessed by the GaAs/AlGaAs material system. However, it is not practical to grow a DBR on InP because the difference of the index of refraction of InP-based DBR pair, such as InGaAs/InAlAs, is relatively small, compared to that of GaAs/AlAs DBR pair. Thus, many more pairs must be grown to achieve high reflectivity. Also, InP substrates are smaller, more fragile, and more expensive than GaAs substrates. There are several approaches to produce long-wavelength emission from heterostructures grown on GaAs substrates: (1) self-assembled InAs/(In)GaAs quantum dots (QDs), (2) dilute nitride GaInNAs(Sb)/GaAs quantum wells (QWs), (3) wafer bonding of InGaAsP/InP active layers to top and bottom GaAs/AlAs DBRs, and (4) type-II InGaAs/GaAsSb QWs. Each approach has its pros and cons. Here we report on the type-II InGaAs/GaAsSb QWs because QWs have more volume for light amplification than QDs, fewer defects than dilute nitrides, and simpler in processing than wafer bonding. The emission wavelength can be tuned independently by the composition and thickness of the electron and hole quantum wells. There are only a few reports on InGaAs/GaAsSb QWs grown on GaAs substrates. InGaAs and GaAsSb are both compressively strained to GaAs, and tensile-strained GaAsP can be grown to compensate InGaAs and GaAsSb. There is one report of MBE-grown strain-compensated GaAsP/GaAs/GaAsSb VCSELs on GaAs, emitting at 1266 nm. So far, there does not seem to be a report on strain-compensated GaAsP/InGaAs/GaAsSb QWs, let alone VCSELs, on GaAs. In this talk we report on molecular-beam epitaxy (MBE) and characterization of these QWs, specifically investigating the effects of strain compensation. The compositions and layer thicknesses are measured by high-resolution X-ray rocking curves and compared with simulations. Two series of samples were grown. One series have 3 InGaAs/GaAsSb samples grown in the same day: without the GaAsP strain-compensating layer, with GaP and with GaAs0.8P0.2 strain-compensating layers. We found that only the last sample with GaAs0.8P0.2 strain-compensating layers shows a strong low-temperature (LT) photoluminescence (PL) at 1100 nm. The second series also have 3 samples with 2, 4, and 6 nm In0.3Ga0.7As thickness while the corresponding GaAs0.8P0.2 thickness is also adjusted to maintain strain compensation and be within its own critical layer thickness. We found that only the 2 nm-thick In0.3Ga0.7As sample shows a strong LT PL at 1050 nm, indicating 4 nm thickness is already larger than the critical layer thickness. These samples do not have room-temperature (RT) PL, but later samples grown with a higher substrate temperature show RT PL. In conclusion, as expected, strain compensation is beneficial, and we are pursuing longer wavelength emission by adjusting the layer parameters through COMSOL and growth conditions.

The Effects of Strain Compensation in Type-II InGaAs/GaAsSb Quantum Wells Grown on GaAs(001) Substrates

I: Novel IR Detector Materials

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EMC 2023

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. 

Charles Tu

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The Effects of Strain Compensation in Type-II InGaAs/GaAsSb Quantum Wells Grown on GaAs(001) Substrates

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