Optimizing Photovoltage in Metamorphic InAs/InGaAs/GaAs Quantum Dot Heterostructures: Design Insights for Light‑Sensitive Devices

Background

Semiconductor nanostructures, especially In(Ga)As QDs, have become cornerstone materials for infrared photonics, offering tunable emission, high efficiency, and cost‑effective fabrication [1–5]. Their versatility spans lasers [2,6], single‑photon sources [7,8], photodetectors [9–13], and solar cells [14–16]. Strategies such as intermediate bandgap excitation [17,18] and multiple exciton generation [19,20] extend photosensitivity and push solar‑cell efficiencies beyond single‑bandgap limits [21]. Strain‑balancing [22] and misfit‑management [23] reduce defect densities, improving device performance [24–26]. Metamorphic buffers (MBs) made of InGaAs lower lattice mismatch and strain in InAs QDs, yielding red‑shifted emission into telecom wavelengths (1.3–1.55 µm) [27–36]. Recent work has explored metamorphic QDs for infrared photodetectors [11–13] and solar cells [37–39], focusing on structure design [25,31–33] and photoelectric enhancement [39,40]. However, the role of the GaAs substrate and its interfaces on PV remains under‑explored, even though these layers can introduce deep levels that affect carrier dynamics [43–45]. This paper extends that work by comparing metamorphic and pseudomorphic QDs with contacts on the buffer or the substrate, elucidating how substrate‑related defects generate bipolar PV and how design adjustments can mitigate these effects.

Methods

Samples were fabricated by MBE on semi‑insulating (si) GaAs (001) substrates (n‑type, 3×10⁷ cm⁻³, 500 µm thick, 2×10⁷ Ω·cm). The metamorphic stack included a 100 nm n⁺ GaAs buffer (600 °C), two In_0.15Ga_0.85As MBs (300 nm and 500 nm, 490 °C), a 3 ML InAs QD layer in a 20 nm undoped In_0.15Ga_0.85As spacer (460 °C), a 300 nm n‑In_0.15Ga_0.85As capping layer (490 °C), and a 13 nm p⁺ In_0.15Ga_0.85As cap (490 °C). The pseudomorphic stack used a 300 nm n⁺ GaAs buffer, a 500 nm n‑GaAs MB, 3 ML InAs QDs in a 20 nm undoped GaAs spacer, and a 500 nm n‑GaAs capping layer. AFM confirmed QD diameters of ~20 nm and heights of ~5 nm for both types [30,31,45]. Mesa structures (500 µm diameter, 400 µm top Au contacts, 70 nm thick) were etched to the n⁺ buffer. Bottom contacts were AuGeNi for buffer‑contacted samples and indium for substrate‑contacted samples. Schottky barriers were enhanced by inserting a thin p⁺ InGaAs layer beneath the Au contact [47–49]. Band‑profile simulations employed Tibercad, incorporating strain, defect traps, and realistic Schottky heights [50,51]. PV and PC spectra were recorded at 300 K with 0.6–1.8 eV excitation (1.5 mW cm⁻²), using a 1 V bias and 100 kΩ load. PL was measured at 532 nm. All contacts were verified Ohmic via linear I–V curves.

Results and Discussion

A. Photoelectric Characterization

Figure 2 shows PV spectra for both QD types. Buffer‑contacted samples exhibit clear signatures of QD ground‑state absorption (0.88 eV for metamorphic, 1.05 eV for pseudomorphic), wetting‑layer transitions (~1.2 eV), and band‑edge absorption of the MB (~1.24 eV). A pronounced decline above 1.36 eV in the metamorphic stack reflects absorption in the heavily doped n⁺ GaAs buffer, whose band non‑uniformities red‑shift the GaAs interband edge [33,46–55]. When the bottom contact is placed on the si‑GaAs substrate, the PV spectra invert below ~1.68 eV (metamorphic) and ~1.44 eV (pseudomorphic), indicating bipolar response. The onset at ~0.72 eV originates from EL2 defect absorption in the substrate or its interface [57]. Thus, substrate‑related defects generate a negative PV component that competes with the positive contribution from QDs, wetting layers, and MBs. Band‑profile simulations (Figure 3) confirm that carriers generated in the n⁺ GaAs/substrate region drift opposite to those generated in the upper layers, producing the observed polarity reversal.

B. Substrate‑Heterostructure Intermediate Layer Design Solutions

Our analysis shows that a standard si‑GaAs substrate combined with a thin n⁺ GaAs buffer is unsuitable for vertical photodetectors, as the space‑charge region at the buffer/substrate interface forces carriers to flow in the opposite direction, diminishing overall efficiency. Potential remedies include: (i) thickening the n⁺ GaAs buffer to shield the substrate (requires >800 nm, still insufficient for full suppression); (ii) inserting a thin (10 nm) undoped Ga_0.3Al_0.7As barrier to confine substrate‑generated electrons (Figure 5b); (iii) adopting a graded or heavily doped n‑GaAs substrate, which aligns band bending with the MBE layers and eliminates the bipolar effect (Figure 5c). The latter approach, used in recent high‑efficiency solar cells [14,39,40], also facilitates a back‑contact design that maximizes light absorption.

Conclusions

Photoelectric measurements reveal that deep levels in the si‑GaAs substrate and its n⁺ buffer produce bipolar PV in both metamorphic and pseudomorphic InAs QD structures. Switching the bottom contact to the substrate removes the negative component, yielding unipolar PV. However, substrate‑related absorption above 1.36 eV still degrades PV and PC. Our simulations and experiments suggest that a heavily doped or graded substrate, optionally buffered with a wide‑bandgap layer, offers the most practical route to eliminate substrate effects and maximize device performance.

Abbreviations

AFM

Atomic force microscopy

MB

Metamorphic buffer

MBE

Molecular beam epitaxy

ML

Monolayer

PC

Photoconductivity

PL

Photoluminescence

PV

Photovoltage

QD

Quantum dot

RL

Load resistance

si

Semi‑insulating

WL

Wetting layer