Two MIS photo-detectors with Ge NCs labeled MIS PD_1 and MIS PD_2 are fabricated with small and large Ge NCs respectively. The insertion of crystalline Ge NCs in the insulator of MIS photodetectors structures aims to enhance the photo-response and the electric transport of the MIS photo-detector. The size of the Ge NCs in photodetectors devices was varied to determine its effect on the photo-response and electric transport in MIS photodetectors.
Figure 3 and 4 display both the schematic structure of innovative MIS PD systems and the morphological and structural characterization of the Ge NCs for MIS PD_1 (small NCs) and MIS PD_2 (large NCs) respectively. The SEM image (Figure 3b) of the small Ge NCs obtained from an amorphous Ge layer of 1 nm reveals the formation of high density Ge NCs (5 1011 NCs/cm2) with homogeneous size (mean size around 7nm). Figure 3c and d shows cross sectional HRTEM images of the NCs embedded in the oxide. As already observed in previous studies [7] the Ge NCs are monocrystalline without visible structural extended defects. The mean diameter ~ 7 nm observed by TEM is in good agreement with the SEM observations.
Figure 4b displays the morphology of the large Ge NCs obtained from a deposition of 2 nm amorphous Ge. The density of Ge NCs is reduced as compared to the previous situation (~ 1011 NCs/cm2). Their size distribution is also broader than for a 1nm deposit. As already observed in the past, the Ge NCs size is controlled by the deposited thickness of amorphous Ge (see [7, 12]). Cross section HRTEM images of the Ge NCs (Fig. 4c and d)evidence monocrystalline NCs free of structural defects that have hemispherical shape with high aspect ratios ~ 0.9. The average diameter of these Ge NCs is ~ 14 nm.
From these morphological and structural characterizations we concluded that the Ge NCs are crystalline, with hemispherical shape and homogenous size in the same order of magnitude than the Bohr radius of Ge. Quantum confinement is then expected to occur and should be considered for the interpretation of the results. Since the Ge NCs size can be tailored with high precision by varying the thickness of amorphous Ge initially deposed on the tunnel oxide, the quantized energy could be controlled by varying the thickness of the deposited amorphous Ge layer.
As mentioned above, the Ge NCs are isolated between two insulating barriers: tunnel (t1 = 5 nm) and capping (t2 = 45 nm) SiO2. These two SiO2 layers are well visible on TEM image (Figs. 3c and 4c). They have been obtained by thermal oxidation (t1) and by PECVD deposition (t2) and they exhibit well-known insulating properties. Therefore, the overall conductivity of the Ge NCs / SiOx matrix system is strongly governed by the properties of the insulator but also by the current through the layer of Ge NCs.
We consider that the conduction current in these structures is continuous and does not depend on the resistance of the system. The latter can be located in the volume of the dielectric or at the interface with the electrodes and corresponds to an extrinsic conduction.
The presence of Ge NCs in insulator affects the transport phenomena in the MIS Photodetector structure. Figure 5a shows the dark current density variation as a function of the applied gate voltage (J-V) for MIS PD_1 (small Ge NCs). The curve is asymmetric with a very large dark accumulation current and a very low dark reverse current as shown at low scale in inset figure in the Fig. 5-a since the transport is governed by the majority carriers which are the electrons because the substrate is N-type doped. To understand the conduction mechanisms in this structure, it is therefore essential to determine all the factors that can affect the electric transport such as the quality and the thickness of the insulating matrix layers. Here, the SiO2 insulating matrix consist of a capping layer (t1 = 45 nm) deposited by PECVD and a thermal layer (t2 = 5nm).
Based on this experimental information, we propose different paths of conduction shown in Fig. 5b to explain the electrical transport mechanisms through the Ge NCs layer. This approach is interesting because it gives information on the effects of structural parameters, considering that the transport of carriers passes mainly through Ge NCs (and not through the traps that may be present in the insulating barriers).
In accumulation regime, at a very low positive gate bias (below 0.3 V), the electrons of the conduction band of the Si substrate cross the tunnel oxide by direct tunnel conduction and afterwards are blocked in the Ge QDs and cannot reach the grid by a direct tunneling process because of the high thickness of the oxide t2, this is why the current resulting from this mode of conduction denoted (I) is very weak. By increasing the positive gate bias; the barrier of the potential of the oxide seen by the electrons of the substrate becomes triangular, the electrons pass from the conduction band of the substrate to the conduction band of the Ge NCs by direct tunnel conduction then pass from the conduction band of the NCs to the grid by Fowler Nordheim type tunnel conduction through an effective thickness less than that of the oxide. this mode of conduction is denoted (II) in Fig. 5-b. By further increasing the positive gate bias, the potential barrier high of the oxide seen by the electrons decrease further and the conduction mode changes from mode denoted (II) to the mode denoted (III and IV) as shown in Fig. 5b and with each passage, the current increases further.
In inversion regime, at low negative gate bias the electrons in the grid get stuck in front of the thick capping oxide t2. The carriers therefore cannot reach the Ge NCs by a direct tunnel process and therefore the current cannot go through the Ge NCs. On the other hand, when the barrier seen by the electrons of the grid becomes triangular at high reverse bias, an injection of the Fowler-Nordheim type is possible. This explains the lower dark current density for reverse bias shown in inset Fig. 5a.
Figure 6 shows the comparison of the dark current density variation for MIS PD_1 (small Ge NCs), MIS PD_2 (larger Ge NCs) and MIS structure without Ge NCs (reference structure), We observe that the current density variation of the tow MIS PD has the same shape than the reference sample, but with a significant increase of the current density. This increase in current density is attributed to the intermediate conduction step created by the Ge NCs which significantly increases the crossing probability of the structure. Similar J-V characteristics were reported previously in MIS structures containing Si NCs and attributed this behavior to hopping conduction through the NCs in the barrier [13–15]. The current density increases as a function of NCs size because when the NCs size increases, the percentage of surface covered by the Ge NCs increases. Such phenomenon was observed on all the structures containing Ge NCs studied in this work (not shown here).
In the light of these findings we can conclude that the presence of Ge NCs in the oxide layer of the MIS photodetector increases the current density in MIS structures due to hopping conduction through the Ge NCs.
In addition, we also performed photocurrent measurements on MIS PD devices. The structures are exposed under normal incidence of white light through the large area of transparent metal electrode 3.2 mm2. When the photons with energy larger than the band gap of the structure illuminate the gate electrode, an electron-hole pair can be generated in Ge NCs or in Si substrate (in the depletion region or in inversion layer) and separated by the built-in electric field. Therefore the generation of electrons in Ge NCs increases and thus the gate current increases. The presence of Ge NCs can then affect the photocurrent generation of electron-holes pairs in the structures as it is explained from the energy band diagram given Fig. 7.
Figure 8 shows the comparison of the current density/voltage (J-V) curves in the dark and under white light illumination for MIS PD with small and large Ge NCs plotted in logarithmic (a-c) and linear (b-d) scales.
For the twos MIS PD devices, there is a strong increase of the forward and reverse photocurrent in presence of Ge NCs under illumination. At forward bias voltage Vg = 1V, the photocurrent density increases from ~ 1 mA/cm2 to ~ 4 mA/cm2 for the MIS PD_1 and increase from ~ 2 mA/cm2 to ~ 7 mA/cm2 for MIS PD_2.
At reverse bias voltage Vg = 1V, the photocurrent density increases from ~ 4 µA/cm2 to ~ 10µ mA/cm2 for the MIS PD_1 and increase from ~ 5 µA/cm2 to ~ 60µ mA/cm2 for MIS PD_2.
We notice that the reverse photocurrent of MIS PD_2 containing large NCs (14 nm) is 10 times higher than that of MIS PD_1 containing small NCs (7 nm).
The observed evolution is in good agreement with the results reported by Shieh et al. and shows that MIS PD_2 represents a good optimization of the structural and optoelectronic properties. The increase of the photoconductive gain due to the presence of Ge NCs is then explained by the large light absorption and electron-hole pair photo-generation thanks to the presence of high density Ge NCs in the structure. Another mechanism which involves an additional flux of electrons to compensate the positive charges accumulated in the Ge NCs layer (holes are accumulated due to their slower conduction from the substrate through the tunnel oxide) could also explain the results since additional electrons should then travel from the AuPd reservoir (to maintain the charge neutrality) through the control SiO2 and contribute to the observed photocurrent. A similar gain mechanism was suggested previously for photoconductive gain in GaN/AlGaN metal-semiconductor- metal PDs where holes are trapped by line defects in GaN. [15, 17]
Figure 9 summarizes the evolution of the current density-voltage (J-V) curves in the dark and under white light illumination for MIS reference and MIS PD _ 2 that have a higher photocurrent (in the rest of this work we only focus on this optimized structure). It is clear that the presence of Ge NCs (14 nm) increases strongly the photocurrent signal at low bias indicating that the devices can be operated at a reverse bias as low as 1 V.
To obtain more information about the spectral response we have performed Photocurrent spectroscopy of the optimized structure MIS PD_2. In the case of MIS PD, this technique is particularly interesting to explore and determine the absorption threshold (and also indirectly the energy gap) associated with the size of the Ge NCs.
Figure 10 displays the photocurrent spectra obtained on MIS PD_2, only including the excitation wavelength lower than 1000 nm (to eliminate any influence of the silicon substrate on the absorption). These results show a broadband absorption at room temperature associated to Ge NCs in the visible range between 400 nm and 950 nm resulting from high energy confinement in the Ge NCs. And a linear increase of photocurrent measured under 680 nm low light power density. These results are very interesting, for improving the spectral response of MIS PD and the realization of high-efficiency PDs that can be easily integrated into a standard silicon complementary metal-oxide semiconductor process. The results also prove that the elaboration method of ultra pure Ge NCs- MIS PD by a combination of MBE deposition and subsequent dewetting is currently the most mature of all published methods.