Comparison between the CE-DWELL detectors with thin and thick

The two samples compared in this section were grown with different CE-DWELL structures as shown in Fig. 6.7. While the thiner AlGaAs layer was thin enough to only cover the QD side walls, the thicker AlGaAs layer was grown to virtually cover the entire QD. Fig.

6.8 shows the TEM image of the test sample with 3 nm AlGaAs layer deposited on the InAs QDs. It confirms the coverage condition of RN0158 as shown in Fig. 6.7. The two different thicknesses of the AlGaAs layers also provide different degrees of confinement enhancement for the DWELL structure. Fig. 6.9 shows the PL spectra of the two samples at 77K. The ground state energy of the thick AlGaAs sample is 27 meV higher than that of the thin AlGaAs sample due to the better quantum confinement from the thicker AlGaAs layer. Such difference in

Fig. 6.7 Schematic diagram of the CE-DWELL structures with thin and thick AlGaAs CE-layers.

Fig. 6.8 Cross-sectional TEM image of the sample with 3 nm AlGaAs layer on the QDs.

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Fig. 6.9 High power PL spectra of the two samples at 77K.

confinement effect is even pronounced for the 2-D like electronic state in the structure because of a more extended wavefunction. The 2-D state has the emission energy 36 meV higher for the thick AlGaAs sample than for the thin AlGaAs sample.

Fig. 6.10 shows the photocurrent spectra of the thin AlGaAs sample and the thick AlGaAs sample as well as the bias dependent responsivity curves at 77K and 200K. Both CE-DWELL samples reveal suitable response band for LWIR detection. The response peak of the thin AlGaAs sample is at 8.2 μm, while that of the thick AlGaAs sample is slightly shorter, at about 8 μm. The thicker AlGaAs layer in the CE-DWELL structure pushed the response peak toward short wavelength as expected. Both of the spectra show small relative bandwidths (Δλ/λp) of about 10% with the B-B type transitions. It should be mentioned that, in the thick AlGaAs sample, the QD fully covered by the AlGaAs layer breaks the symmetry of the QD confinement in DWELL structure along z-direction. Fig. 6.11 shows the photocurrent spectra of the thick AlGaAs sample under different bias polarities. Different from the case for the thin AlGaAs sample (see Fig. 6.2(b)), the photocurrent spectrum of the thick AlGaAs sample shows different shapes for the positive and negative bias conditions. The single peak structure at negative bias split into two peaks when the bias polarity changed to be positive. Comparing the responsivity of the two samples (Fig. 6.10), the thin AlGaAs sample shows higher reponsivity

98 

at both temperatures. At 200K, it reaches 0.37 A/W at 0.8V. Although there is a stronger confinement effect in the sample with thicker AlGaAs layers, the responsivity instead degraded.

However, its responsivity (37 mA/W at -1.6V at 77K) is still higher than that of the sample without AlGaAs layers (23 mA/W for RN0176 under the same electric field), showing the advantage of the confinement-enhanced DWELL structure.

Fig. 6.10 The voltage dependence of the peak responsivity of the two samples at 77K and 200K. The inset shows the responsivity spectra of the two samples at -1.35V and 77K.

Fig. 6.11 The photocurrent spectra of the thick AlGaAs sample under positive bias and negative bias at 77K.

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To understand the origin of the inferior responsivity in the thick AlGaAs device, the comparison of current gain for the two samples at 77K and 200K is shown in Fig. 6.12. Clear degradation of current gain is shown in the thick AlGaAs sample due to the thicker AlGaAs layers. Furthermore, the two samples show distinct gain behaviors. The current gain of the thick AlGaAs sample possesses a clear asymmetry between the positive bias and the negative bias. Because of thicker AlGaAs layers used, the QDs in the thick AlGaAs sample are virtually covered. When positively biased, the photo-generated electrons have to overcome the potential barrier of AlGaAs and pass through the InGaAs well. The capture probability of the excited carriers into the adjacent InGaAs well is high when the bias is low. Higher positive voltages are needed to reach the same current gain as that in the reverse bias. On the other hand, in the thin AlGaAs sample, because of thinner AlGaAs layers, the current gain is more symmetric under different bias polarities and it has a much higher gain than that of the thick AlGaAs sample under the same positive bias. The current gain difference of the two samples also depends on the device temperature. At higher temperatures, because of the additional thermal energy, the capture probability of electrons decreases in both bias polarities. The gain increases in both samples and the asymmetry reduces in the thick AlGaAs sample.

Fig. 6.12 The current gain curves of the two samples at 77K and 200K.

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The decrease of responsivity in the thick AlGaAs sample cannot be only explained by the gain degradation. In fact, the QE also degraded for the thick AlGaAs sample. To further investigate the QE, the polarization-dependent photo-response of our devices was measured using the 45^o edge coupling scheme. The results are shown in Fig. 6.13(a) and (b) for the two samples. The spectra changed differently for the two samples as the polarizer angle varied from 0 degree (induce mixed TM and TE response) to 90 degree (induce pure TE response). For the thin AlGaAs sample, the TE response is ~49% of the TM response. While for the thick AlGaAs sample, the TE to TM response ratio is only ~28%. A stronger absorption or higher QE is expected for the thin AlGaAs sample when the radiation is normal to the surface of the devices.

The better normal incident absorption (TE response) for the thin AlGaAs sample comes from a better lateral confinement of carriers in QDs. Because the AlGaAs barrier layer is thin enough to leave the tips of the QDs uncovered, this additional barrier is mainly in the lateral direction. This lateral confinement enhances normal incident absorption as mentioned in chapter 5. For the thick AlGaAs sample, the tips of the QDs are covered by the thick AlGaAs layer. Since the vertical confinement is also enhanced in this case, the advantage from the lateral confinement is reduced. This explains why the QE of the thin AlGaAs sample is better than that of the thick AlGaAs sample under the normal incidence configuration. This also shows the importance of having a proper AlGaAs layer thickness in the devices.

A Fig. 6.13 The polarization-dependent photocurrent spectra of (a) the thin AlGaAs sample and (b) the thick lGaAs

sample at 40K under the 45^o facet-coupled configuration.

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102 

The AlGaAs la s too thick from the

responsivity point of view, is eff

bette yer in the thick AlGaAs device, although i

ective to reduce the dark current. This is especially true at high temperatures and low bias voltages. The dark current density for the thick AlGaAs sample and the thin AlGaAs sample are 1.4×10^-8 A/cm² and 4.1×10^-6 A/cm² respectively at -1V and 77K.

The thicker AlGaAs layers effectively block the low energy part of the dark current, so the detectivity of the thick AlGaAs device is not necessarily worse than that of the thin AlGaAs device. Fig. 6.14 shows the specific detectivity of the two samples at different temperatures. At 77K, the highest measured detectivity for the thin AlGaAs sample is 3.6×10¹⁰ cmHz^0.5/W at 1.4V. However, as the temperature increases, the thick AlGaAs sample starts to show r performance. At 220K, the highest detectivity measured for the thick AlGaAs sample is 4.85×10⁷ cmHz^0.5/W at -0.15V, which is about 2.4 times higher than that of the thin AlGaAs sample. When the temperature is further raised to 240K, the detectivity reaches 1.4×10⁷ cmHz^0.5/W at -0.1V for the thick AlGaAs sample, while for the thin AlGaAs sample the dark current is too high for the measurement.

Fig. 6.14 The specific detectivity curves of the two samples at 77K, 120K, 160K a d 220K.

The comparison of the two samples shows an important factor for the high temperature

n

103 

operatio

.4 Comparison between the CE-DWELL detectors with Al

0.2

Ga

0.8

As and

ntent in AlGaAs layer for the CE-DWELL structure

n. The thin AlGaAs sample is a typical high performance detector at 77K with good carrier collection capability. However, as the temperature increases, the superior carrier transport property induces high dark current which is not acceptable in real applications. On the other hand, although the insertion of the thicker AlGaAs layer in the thick AlGaAs sample degrades the responsivity, the dark current is suppressed even more especially at small biases.

As the temperature increases, the thermal energy helps the photocarrier collection and provides a better performance at higher temperatures. This indicates the importance of the current suppression for the high operating temperature devices.

6

with Al

0.3

Ga

0.7

As CE-layers

In principle, to increase the Al co

can increase the confinement state energies accordingly. However, the epitaxial process of the QDs makes the issue more complicated. A series of single layer CE-DWELL structure was prepared with different Al content in the AlGaAs layer ranging from 0% to 30%. Fig. 6.15 shows the PL spectra of the samples as well as the PL spectrum of the conventional DWELL structure. Surprisingly, opposite trend of the ground state energy shift is clearly shown in the figure. The sample with the weakest confinement (i.e. the sample with GaAs layer) generates the highest ground state energy. The intermixing effect during the epitaxial process dominates the behavior. As published, strong intermixing of the Ga atoms with InAs QDs is observed when the InAs QDs are covered by the GaAs layer [6.14-6.15]. With the increase of the Ga content in the QD, the ground state energy is thus raised. When the Al content in AlGaAs layer increases, the intermixing effect decreases and the ground state energy is thus lower. When the Al content is around 20%, the decrease of intermixing effect and the increase of the quantum confinement are balanced. No obvious ground state energy shift is observed when the Al content is higher than 20%.

Fig. 6.15 The PL spectra of the samples with different Al content in the AlGaAs confinement enhancing layer. The

However, the device performance can different even though the state energies are

PL spectrum of the conventional DWELL structure is also in luded. c

be

almost the same. In principle, lower Al content (20%) is preferred from the material quality point of view, but higher Al content (30%) is preferred from the wavefunction confinement viewpoint. To find out the better solution, two device samples with Al0.3Ga0.7As (RN0615) and Al0.2Ga0.8As (RN0613) confinement enhancing layers were compared. Fig. 6.16(a) shows the

Fig. 6.16 (a) Schematic diagram of the CE-DWELL QDIPs and (b) the 3-D AFM image of the sample RN0615.

104 

105 

sam

te

sed

PL spectrum of the sample (both the Al0.3Ga0.7As sample and the Al0.2Ga0.8As sample show v

ple structure. In the two samples, the thickness of the In0.15Ga0.85As QW above the AlGaAs layer was reduced to 2.8 nm to reduce the strain energy accumulation. Moreover, the growth mperature for QDs was lowered to 495^oC to improve the QD density. Fig. 6.16(b) shows the AFM image of the sample. With the reduced growth temperature, the dot density was increa to ~4x10¹⁰ cm^-2, about 1.6 times higher than for the samples in previous sections. The lower growth temperature also caused reduced In-Ga intermixing. Fig. 6.17(a) shows the high power

ery similar results). Compared with the sample in section 6.2 (see Fig. 6.2), the ground state energy for the samples here is lowered by 25 meV due to the reduced intermixing of Ga atoms with InAs QDs. More than that, with the reduced intermixing, the energy separations between QD states are also enlarged and therefore, the infrared response shifts to a shorter wavelength of 7.2 μm as shown in Fig. 6.17(b). Again, the response peak corresponds to the transition between the ground state (1.067eV) and the 4^th excited state (1.324eV) of the QDs assuming that ΔEc : ΔEv ≒ 2:1. With the thin enough AlGaAs layer, the symmetry of the QD confinement barriers along z-direction is also preserved and the spectral shapes under different bias polarities are nearly the same (Fig. 6.17(b)).

Fig. 6.17 (a) The high power PL spectrum and (b) the photocurrent spectra with different bias polarities of the sample RN0615.

106 

Fig. 6.18 shows K and 140K. It is clear

seen the lower barrier of sponsivity due to the batter

carrier transport property 2Ga0.8As sample reaches

1.2 A/W, which is about 2.4 ti As sample. Fig. 6.19(a)

shows their comp r effective barrier height,

the Al0.2Ga0.8As samp e Al0.3Ga0.7As sample,

th

concentrated electron wavefunctions. The QE is thus higher for the sample with Al0.3Ga0.7As the responsivity curves for the two samples at 77

Al0.2Ga0.8As layers does improve the re . At 77K and 1V, the responsivity of the Al0.

mes higher than that of the Al0.3Ga0.7

arison for the QE and the current gain. With the lowe le indeed shows higher current gain. However, in th

e suppression of the intermixing effect gives a smaller effective QD size and more

layers. The result also suggests that the material quality does not suffer severely for the thin enough AlGaAs layer grown on QDs even with a higher Al content. The highest QE measured

Fig. 6.18 The voltage dependence of responsivity of the two samples at different temperatures.

Fig. 6.19 The voltage dependence of (a) quantum efficiency, current gain and (b) detectivity of the two samples.

107 

for the A revious

samples due to the im ith the lower dark

current, the sam expected (Fig.

6.19(b)). At 77K, the peak Hz^0.5/W at 1.1V,

proved QD density. With the higher QE, combined w ple with Al0.3Ga0.7As layers also revealed higher detectivity as

detectivity of the Al0.3Ga0.7As sample is 4.7×10¹⁰ cm es higher than that of the Al0.2Ga0.8As sample.

to further reduce the dark current and elevate the operation tem

3Ga0.7As CE-layers, another sample with an additional 50 nm Al current blocking layer [6.16] just above the bottom contact layer was al

ig. 6.20 shows the photocurrent spectrum at 140K and the responsivity curves with different temperatures of the sample. With the inserted 50 nm Al0.25Ga0.75As current blocking layer, the device performance at higher temperatures was indeed improved. The photocurrent spectrum measured by FTIR showed a clear signal at around 7.1 μm even at 140K. While for the sample without such blocking layer (RN0615), the spectrum can be measurem nt with acceptable signal quality up to only ~125K. As expected, the inserted AlGaAs lay

located far from the QDs did not influence the detection wavelength much. The responsivity

Fig. 6.20 The voltage dependence of responsivity of the sample at 77K, 140K and 170K. The photocurrent spectrum is also included in the insert.

108 

curve can be m the responsivity curve

reveals a clear asymm fect of the

single-sided blocking layer r QDs is injected from

the bottom Ga0.75As layer so the

photo-response suff ple at different

temperatures. Wh

faster for the positive biases th rior responsivity under

the positive bias condition. At 170K, the peak detectivity at the negative bias is 2.3×10⁷

cm in

easured up to 170K with the lock-in technique. At 170K,

etric behavior for different bias polarities showing the ef . At positive bias, the carrier replenishment fo

contact and is readily hindered by the 50 nm Al0.25

ers. Fig. 6.21 shows the measured detectiviy for the sam

en the temperature is raised to higher than 160K, the detectivity decreases an for the negative biases due to the infe

Hz^0.5/W, which is about 2 times higher than for the positive bias. However, the asymmetry the device characteristics is not clear for the temperature lower than 140K. It is therefore expected that the device performance can be further improved by using a higher barrier current blocking layer. In this way, a more pronounced asymmetric operation condition with a higher operating temperature at the negative bias is expected.

Fig. 6.21 The voltage-dependent specific detectivity of the sample at different temperatures.

109 

6.5 Summary

In summary, we performed the detailed studies on the QDIPs with confinement-enhanced dots-in-a-well (CE-DWELL) structures. A thin AlGaAs layer was inserted on top of the QDs in the DWELL structure to enhance the wavefunction confinement and device performance. With proper device parameters, LWIR QDIPs with operation temperatures higher than 200K and detection wavelength of 8.2 μm were achieved for the first time. Two CE-DWELL structures with different thicknesses of Al0.3Ga0.7As CE-layers were investigated. The device with thin AlGaAs uch higher normal-incident absorption capability and superior 77K device performance. On the other hand, thick AlGaAs layers greatly elevate the dark current activation energy so that the device, although unfavorable for low temperature operations, showed much higher detectivity at high realization of high temperature operation QDIPs, better suppression of the

. o

As layers showed a better layers showed m

temperatures. To the

dark current is the key Tw other CE-DWELL detectors with Al0.3Ga0.7As CE-layers and Al0.2Ga0.8As CE-layers were also compared. Although the state energy distribution was nearly the same due to the balanced intermixing effect and quantum confinement effect, the device performance was very different for the two samples. The Al0.3Ga0.7As layer provided stronger barrier confinement for the QDs and thus resulted in higher quantum efficiency for QDIPs.

Although the current gain was sacrificed, the device with Al0.3Ga0.7

overall performance than the device with Al0.2Ga0.8As layers. By employing a proper current blocking layer to suppress the dark current, the device operating temperature can be further elevated.

110 

InAs/GaAs Quantum Ring Infrared Photodetectors

tocurrent spectra, more stable responsivity with temperature change, and lower dark urrent activation energy. The wide detection band comes from the transitions from the QR round states to different excited states. The shallow confinement states generate higher dark urrent and enhance the carrier flow between the QRs within the same QR layer. This carrier ow partly smoothes over the repulsive potential and makes QRIPs behave similarly to QWIPs stead of QDIPs. With an Al0.27Ga0.73As current blocking layer, the operating temperature of QRIPs is greatly raised.

possess distinct characteristics because of the differing quantum onfinement in their structures. In quantum wells (QWs), the electrons are confined within the

Chapter 7

In this chapter, InAs quantum rings (QRs) are used as the absorption media in infrared photodetectors and the device characteristics are investigated under normal incidence configuration. Compared with QDIPs, quantum ring infrared photodetectors (QRIPs) show wider pho expected to be the economical alternatives to the II-VI based interband detectors for the implementation of large format imaging focal plane arrays. With this motivation, quantum well infrared photodetectors (QWIPs) and quantum dot infrared photodetectors (QDIPs), especially for the n-type devices, were widely studied with different spectral ranges in past decades [7.1-7.14]. With the intersubband transitions, the fractional spectral widths (Δλ/λ) of the photocurrent spectra are usually smaller than 30% in both QWIPs and QDIPs. However, these two kinds of detectors

c

111 

otential wells in the vertical direction but are free to move along the wells. As a result, QWIPs re only sensitive to radiations

p

a with the electric field polarized perpendicular to the quantum ent conductive gain of QWIPs is usually low and quite stable at different wells [7.2]. The curr

temperatures[7.1]. In contrast, with the three-dimensional quantum confinement for the active media, QDIPs exhibit strong in-plane polarized photo-response [7.12-7.14], much higher current gain, and strong temperature dependence of responsivityas discussed in section 6.2.

With the advances in nanostructure growth, e.g. the growth of ring-like structures as described in chapter 3, it is expected that new structures with different confinement geometry will

With the advances in nanostructure growth, e.g. the growth of ring-like structures as described in chapter 3, it is expected that new structures with different confinement geometry will

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